[media] rcar-vin: rename entity to digital

When Gen3 support is added to the driver more then one possible video
source entity will be possible. Knowing that the name entity is a bad
one, rename it to digital since it will deal with the digital input
source.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>