Add I2S Slave Support For Teensy 4.0

[bthayer3]

Looks like the AudioInputI2Sslave_F32 is targeting an older teensy 3 board. Add a new logic section to support Teensy 4.0 board. I compared the logic used in the PJRC audio library and added this logic more so as an educated guess. It my testing it does work with some glitches though, I am probably testing this all wrong though as I don't have an oscilloscope.

Probably would also need the design tool with this i2sslaveinput drag and drop element. Didn't add that code yet as I think this slave code needs more work.

For reference: this is the code I was using in the Arduino sketch when I tested this. I was sending over 16 bit at 48 khz:

#include "OpenAudio_ArduinoLibrary.h"
#include "AudioStream_F32.h"
#include "USB_Audio_F32.h"
#include <Audio.h>
#include <Wire.h>
#include <SPI.h>
#include <SD.h>
#include <SerialFlash.h>

AudioInputI2Sslave_F32       audioInI2S1;
AudioOutputUSB_F32           audioOutUSB1;
AudioConnection_F32          patchCord1(audioInI2S1, 0, audioOutUSB1, 0);
AudioConnection_F32          patchCord2(audioInI2S1, 1, audioOutUSB1, 1);

void setup() {
  AudioMemory(30);  
  AudioMemory_F32(30);
}

void loop() {
}

[boblark]

I confess to being unqualified in this area of the code! So some random thoughts.
I think we can get 16-bit code by borrowing from the work of the I16 library. But, that leaves up to 8-bits that may be available and within the dynamic range of floating point. We also have this issue with the non-slave quad input as well. Does this seem right?
Alternatively, we can restrict the slave input to 16-bits resolution?
The need for slave seems less with 4.1 than 3.6 as the I2S clock generation is both more accurate and can do all the frequencies we need, I think. Thoughts?
Bob