media: atomisp: ov2680: Add exposure and gain controls

Add exposure and gain controls. This allows controlling
the exposure and gain through standard v4l2 IOCTLs.

Note the register defines for the exposure and gain registers
are renamed to match the datasheet.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>