Add gpiokeys driver

gpiokey driver implements functional subset of gpiokeys device-tree bindings:
https://www.kernel.org/doc/Documentation/devicetree/bindings/input/gpio-keys.txt

It acts as a virtual keyboard, so keys are visible through kbdmux(4)

Driver maps linux scancodes for most common keys to FreeBSD scancodes and
also extends spec by introducing freebsd,code property to specify
FreeBSD-native scancodes.

Reviewed by: mmel, jmcneill
Differential Revision: https://reviews.freebsd.org/D6279