rpi-power-switch: Add power switch module

This adds a module to allow users to emulate a power switch using
a GPIO.  It supports either switch operation, which will turn the
device "off" when the switch is flipped, and will restart it when
the switch is flipped back, or button operation, so it works in a
push-push fashion (i.e. one push shuts the board down, another push
turns it back on.)