media: atmel: microchip-csi2dc: add link validation support

With this, the csi2dc will take part in the media pipeline graph walk
and validate the links with it's entities.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>