media: adv7180: implement g_register and s_register

media: adv7180: implement g_register and s_register

The g_register and s_register callbacks are useful for debugging the
adv7180.

Implement the callbacks to expose the register debugging to userspace.

Signed-off-by: Thorsten Schmelzer <tschmelzer@topcon.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>