gpio: Add gpio-fsm driver

The gpio-fsm driver implements simple state machines that allow GPIOs
to be controlled in response to inputs from other GPIOs - real and
soft/virtual - and time delays. It can:
+ create dummy GPIOs for drivers that demand them,
+ drive multiple GPIOs from a single input, with optional delays,
+ add a debounce circuit to an input,
+ drive pattern sequences onto LEDs
etc.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>