issue #536 CPX change SPI pin designations to externally available pins

[brentru]

Reassigned A1, A2, A3 to MOSI/MISO/SCK. Removed FLASH_CS.
#536

[dhalbert]

Oops, that particular assignment doesn't work due to the limited pad assignments on the SERCOM:

>>> spi = busio.SPI(clock=board.A3, MOSI=board.A1, MISO=board.A2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: MOSI and clock pins incompatible

This is what works and doesn't work (copied from REPL trials):

>>> spi = busio.SPI(clock=board.A3, MOSI=board.A2, MISO=board.A1) #OK
>>> spi = busio.SPI(clock=board.A1, MOSI=board.A3, MISO=board.A2) #OK

>>> spi = busio.SPI(clock=board.A1, MOSI=board.A2, MISO=board.A3)  # incompatible
>>> spi = busio.SPI(clock=board.A3, MOSI=board.A1, MISO=board.A2)  # incompatible
>>> spi = busio.SPI(clock=board.A2, MOSI=board.A1, MISO=board.A3)  # incompatible (A2 can't be clock)
>>> spi = busio.SPI(clock=board.A2, MOSI=board.A3, MISO=board.A1)  # incompatible (A2 can't be clock)

@ladyada, do you have a preference? No silkscreen now but there may be in the future. Also may need to consider another possible microcontrollers in same form factor (though that's mostly a routing issue).

[ladyada]

we wont be putting the silkscreen for SPI onto the CPX, and on the classic, SPI isnt exposed, so....use any pins ya like (other than A0 of course :)

[brentru]

@dhalbert Ok - switched pin assignments to mirror your second trial on the REPL.

[dhalbert]

Great! Thanks. We'll need to update the Learn Guide later to mention this when 3.0 comes out.