Add the GPIO driver for the ADI Engineering RCC-VE and RCC-DFF/DFFv2.

This driver allows read the software reset switch state and control the
status LEDs.

The GPIO pins have their direction (input/output) locked down to prevent
possible short circuits.

Note that most people get a reset button that is a hardware reset. The
software reset button is available on boards from Netgate.

Sponsored by: Rubicon Communications (Netgate)