This commit adds support for generating named pin mappings for all pins
including CPU, board-defined, LED and externally controlled pins. CPU pins
are mapped to `pin_GPIO<n>`, externally-controlled pins are mapped to
`pin_EXT_GPIO<n>`, and defined conditionally (up to 10 pins, and can be
expanded in the future), and they are non-const to allow `machine-pin.c` to
write the pin object fields. Both CPU and externally controlled pins are
generated even if there's no board CSV file; if one exists it will just be
added to board pins.
Handle externally controlled GPIO pins more generically, by removing all
CYW43-specific code from `machine_pin.c`, and adding hooks to initialise,
configure, read and write external pins. This allows any driver for an
on-board module which controls GPIO pins (such as CYW43 or NINA), to
provide its own implementation of those hooks and work seamlessly with
`machine_pin.c`.
