overlays: Rename pi3- overlays to be less model-specific (#3052)

Rename the various pi3- overlays to be more generic, listing
the devices they apply to in the README. The original names are
retained for backwards compatibility as files that just include
the new versions - the README marks them as being deprecated.

See: https://github.com/raspberrypi/firmware/issues/1174



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