media: i2c: imx214: Add analogue/digital gain control

The imx214 sensor supports analogue gain up to 8x and digital gain up to
16x. Implement the corresponding controls in the driver. Default gain
values are not modified by this patch.

Acked-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: André Apitzsch <git@apitzsch.eu>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>