media: rkisp1: resizer: Simplify register access

The registers for the mainpath and selfpath resizers are located at the
same offset from the instance-specific base. Use this to simplify
register access, removing the need to store per-register offsets in the
rkisp1_rsz_config structure.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>