[media] media: adv7604: document support for ADV7612 dual HDMI input decoder

This documentation accompanies the patch adding support for the ADV7612
dual HDMI decoder / repeater chip.

Signed-off-by: Ian Molton <ian.molton@codethink.co.uk>
Reviewed-by: William Towle <william.towle@codethink.co.uk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>