overlays: Add gpio-hog overlay

Add an overlay to activate a hog on a specified pin. Note that
hogged GPIOs are not available for other uses, i.e. drivers or
gpioset/gpioget.

Inspired by a Forum thread here:

    https://forums.raspberrypi.com/viewtopic.php?t=334286



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