media: atmel: atmel-isc: create register offsets struct

Create a struct that holds register offsets that are product specific.
Add initially the CSC register.
This allows each product that contains a variant of the ISC to add their
own register offset.

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+huawei@kernel.org>