Add the light frequency control to be able to set the frequency
to manual (50Hz or 60Hz) or auto.

[Sakari Ailus: Rename "ctl" as "ctrl" as agreed.]
[mchehab+samsung@kernel.org: fixed two coding style warnings]
Signed-off-by: Mylène Josserand <mylene.josserand@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>