expose all mprj_io pins in the micropython HAL

[proppy]

Slightly related to #65, but assuming we're using the default micropython firmware it seems that only a subset of the caravel mprj_io are exposed in the micropython HAL (despite all of them being physically connected to CN11 and CN12):
https://github.com/micropython/micropython/blob/05bb26010e4a466a82cfed179f8d8d0b406a78ca/ports/stm32/boards/NUCLEO_F746ZG/pins.csv

We should make sure that all of those are expose to the micropython environment, so that people can poke at their design directly from the micropython REPL.

[proppy]

/cc @mithro

[proppy]

Found out that most of the pins are actually exposed, you just need to remove the P prefix, i.e:
IO[6] -> PE0 -> machine.Pin('E0')