Enable use of pins >32 on RP2350B

[jepler]

Tested on Adafruit Metro RP2350 prototype, which uses pins 32/33. Also requires some small changes to CircuitPython but never mind that :)

>>> import usb_host, board, digitalio, usb
>>> usb_host.Port(board.USB_HOST_DATA_PLUS, board.USB_HOST_DATA_MINUS)
>>> pwr = digitalio.DigitalInOut(board.USB_HOST_5V_POWER)
>>> pwr.switch_to_output(True)
>>> list(usb.core.find(find_all=True))
[<Device>]

ping @ladyada

Closes: #133

[jepler]

I'll update this branch and make it work with the CI. What SDK versions need to be supported? I think SDK 2.1.0 may be required for some of the APIs used, so I can make them conditional if necessary.

[jepler]

CI now runs against SDK 2.1.0 as well as older versions.