Config gpios UX needs improving

[SpComb]

The current CONFIG_TYPE_UINT16 with just GPIO_NUM_MAX has a bad UX for configuring GPIO pins - the pin numbers are meaningless without reference schematics, and it is too easy to clobber some important e.g. IO0 -> RMII_CLK pin.

The GPIO pins should use an enum-style config, which meaningful IO pin names set by the boardconfig.

This will require some additional config support for I2C-GPIO devices.