bcm2835-virtgpio: Virtual GPIO driver

Add a virtual GPIO driver that uses the firmware mailbox interface to
request that the VPU toggles LEDs.

gpio: bcm-virt: Fix the get() method

The get() method does not understand the on-the-wire encoding of the
remote GPIO states, thinking they are simple on/off bits when they are
really pairs of 16-bit counts. Rewrite the get() handler to return the
value last written, which will eventually match the actual GPIO state
if there are no other changes.

See: https://github.com/raspberrypi/linux/issues/4638



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