[media] media: adv7604: ability to read default input port from DT

Adds support to the adv7604 driver for specifying the default input
port in the Device tree. If no value is provided, the driver will be
unable to select an input without help from userspace.

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