Builds fail if variant does not have any GPIOs

[DhruvaG2000]

Add conditional compilation for all relevant GPIO implementations, so that in the absence of GPIOs the builds don't just fail.
This will especially help in adding variants like am62x_m4 that don't yet have GPIO support.

[DhruvaG2000]

As part of cleanup, I think each variant should have it's own config based on what features it is supporting and we should obey that config