media: rcar-vin: Rename 'digital' to 'parallel'

As the term 'digital' is used all over the rcar-vin code in place of
'parallel', rename all the occurrencies.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>