media: ar0521: Add V4L2_CID_ANALOG_GAIN

Add support for V4L2_CID_ANALOG_GAIN. The control programs the global
gain register which applies to all color channels.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>