media: i2c: ds90ub953: Move reg defines to a header file

Move UB953 register defines to a header file. This is done so that the
deserializer driver can access the defines, and do some early
serializer configuration.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Jai Luthra <jai.luthra@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>