media: imx274: rearrange sensor startup register tables

Rearrange the imx274_start_<N> register tables to better match the
datasheet and slightly simplify code:

 - collapes tables 1 and 2, they are applied one after each other and
   together they implement the fixed part 1 of the startup procedure
   in the datasheet
 - while there, cleanup comments
 - rename tables 3 and 4 -> 2 and 3, coherently with the datasheet

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>