This commit cleans up the pin generation code, all the pins and their
AF (only ADC, for now) are specified in nrf52_af.csv and board use their
own csv file to specify which pins are available on that board and if
they have any special names.
Before this change, `microcontroller.cpu.uid` returned values
where the top 4 bits of each byte were zero, because of
an incorrect bitmask used in this function.
shared_bindings/index.rst: updated Support Matrix format as discussed in PR #503 & Issue #448.
shared-bindings/microcontroller/Processor.c & .h: added UID lookup functionality for use with all ports. Fixes#462.
