media: atomisp: ov2680: Add test pattern control

Add a test pattern control. This is a 1:1 copy of the test pattern
control in the main drivers/media/i2c/ov2680.c driver.

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>