esp32,stm32: Add new machine.I2S class for I2S protocol support.

This commit adds I2S protocol support for the esp32 and stm32 ports, via
a new machine.I2S class.  It builds on the stm32 work of blmorris, #1361.

Features include:
- a consistent I2S API across the esp32 and stm32 ports
- I2S configurations supported:
  - master transmit and master receive
  - 16-bit and 32-bit sample sizes
  - mono and stereo formats
  - sampling frequency
  - 3 modes of operation:
    - blocking
    - non-blocking with callback
    - uasyncio
  - internal ring buffer size can be tuned
- documentation for Pyboards and esp32-based boards
- tested on the following development boards:
  - Pyboard D SF2W
  - Pyboard V1.1
  - ESP32 with SPIRAM
  - ESP32

Signed-off-by: Mike Teachman <mike.teachman@gmail.com>

ports/stm32/main.c

@@ -527,6 +527,10 @@ soft_reset:
     pyb_usb_init0();
     #endif
 
+    #if MICROPY_HW_ENABLE_I2S
+    machine_i2s_init0();
+    #endif
+
     // Initialise the local flash filesystem.
     // Create it if needed, mount in on /flash, and set it as current dir.
     bool mounted_flash = false;