Initial support for the Blokas Pisound

This commit adds a driver for the Blokas Pisound audio/MIDI HAT based on
the Linux sources.

Currently this requires a patch to Circle to allow the Pi's I2S audio
device to operate in slave mode.

The Pisound only works at 48000Hz, 96000Hz, and 192000Hz. The Pisound is
automatically detected by the startup code; the user only has to enable
I2S audio output.

MIDI output and recording functionality is not supported.

Closes #42.

README.md

@@ -48,6 +48,7 @@
     + [clumsyMIDI](#clumsymidi)
     + [mt32-pi-midi-hat](#mt32-pi-midi-hat)
     + [PI-MIDI](#pi-midi)
+	+ [Pisound](#pisound)
 - [💬 Custom System Exclusive messages](#-custom-system-exclusive-messages)
 - [❓ FAQ](#-faq)
 - [⚖️ Disclaimer](#%EF%B8%8F-disclaimer)
@@ -160,6 +161,7 @@ The following models of DAC have been confirmed as working by our testers. Pleas
 | Manufacturer | Device             | DAC chip | Additional config file options                         | Comments                                                                                                               |
 |--------------|--------------------|----------|--------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------|
 | Arananet     | [PI-MIDI]          | UDA1334  | None required                                          | Stereo RCA output. Custom design by @arananet also with GPIO MIDI in. Tested by @dwhinham.                             |
+| Blokas       | [Pisound]          | PCM5102A | Ensure `sample_rate` is set to 48000, 96000, or 192000 | Stereo 6.35mm output, volume knob, and integrated SPI MIDI interface. Tested by @dwhinham and @sigkill.                |
 | chris-jh     | [mt32-pi-midi-hat] | PCM5102A | None required                                          | Stereo 3.5mm output. Custom free/open source design by @chris-jh also with GPIO MIDI in. Tested by @gmcn42.            |
 | Generic      | [GY-PCM5102]       | PCM5102A | None required                                          | Stereo 3.5mm output. Found very cheaply on AliExpress and other sites. Tested by @dwhinham.                            |
 | Generic      | [Pi-Fi DAC+ v2.0]  | PCM5122  | `i2c_dac_init = pcm51xx`, `i2c_dac_address = 4d`       | Stereo RCA and 3.5mm output. Tested by @rc55.                                                                          |
@@ -355,6 +357,14 @@ The [PI-MIDI] by @arananet is a pre-assembled device which provides an OLED disp
 
 <br clear="right"/>
 
+### Pisound
+
+[<img width="180rem" align="right" src="docs/blokas_pisound.jpg">][Pisound]
+
+The [Pisound] by Blokas is a commercial Raspberry Pi sound card with DAC, ADC (recording device) and MIDI interface, and is the closest product available to a "professional" audio device. Currently, only the MIDI input and audio output is supported by `mt32-pi`.
+
+<br clear="right"/>
+
 ## 💬 Custom System Exclusive messages
 
 `mt32-pi` responds to MT-32 SysEx messages as you would expect, but it can also respond to its own commands. `mt32-pi` listens for manufacturer ID `0x7D`, or the "non-commercial/educational use" special ID. Therefore, a complete `mt32-pi` SysEx message looks like:
@@ -426,6 +436,7 @@ This project, just like [Munt], has no affiliation with Roland Corporation. Use
 [Pi-DAC Pro]: https://web.archive.org/web/20191126140807/http://iqaudio.co.uk/hats/47-pi-dac-pro.html
 [Pi-Fi DAC+ v2.0]: https://www.aliexpress.com/item/32872005777.html
 [PI-MIDI]: https://www.arananet.net/pedidos/product/pi-midi-a-baremetal-mt32-emulator-using-raspberry-pi3
+[Pisound]: https://blokas.io/pisound/
 [Raspberry Pi power supply page]: https://www.raspberrypi.org/documentation/hardware/raspberrypi/power/README.md
 [REC002004B]: https://www.raystar-optronics.com/oled-character-display-module/REC002004B.html
 [Releases]: https://github.com/dwhinham/mt32-pi/releases/latest