overlays: Add a sample hat_map

The HAT map is way of associating named overlays with HATs whose
EEPROMs were programmed with the contents of the overlay.
Unfortunately, change in the DT and kernel drivers has meant that some
of these embedded overlays no longer function, or even don't apply.

The HAT map is a mapping from HAT UUIDs to overlay names. If a HAT with
a listed UUID is detected, the embedded overlay is ignored and the
overlay named in the mapping is loaded in its place.

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