media: ti-vpe: cal: Don't modify cal_csi2_phy base_fields

In preparation to constify cal_csi2_phy, avoid modifying the base_fields
array at runtime. As the array now only needs to stored two values
instead of a full struct reg_field instance, save memory by using a
custom structure for the base_fields elements.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>