NUM_GPIOS amd NUM_EXT_GPIOS are currently hardcoded in make-pins.py, which
makes it difficult to support SoCs with different pin count.
This commit generalises make-pins.py by passing in the pin count in via the
new arguments `--num-gpios` and `--num-ext-gpios`. These default to the
current values supported by Pico, namely 30/10. This can be changed with
PICO_NUM_GPIOS and PICO_NUM_EXT_GPIOS in `mpconfigboard.cmake`.
Signed-off-by: Damien George <damien@micropython.org>