media: cec-gpio: support 5v testing

Add support for the new (optional) 5V gpio in order to debug 5V
changes. Some displays turn off CEC if the 5V is not detected,
so it is useful to be able to monitor this line.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>