media: ov7670: add V4L2_CID_TEST_PATTERN control

The ov7670 has the test pattern generator features.  This makes use of
it through V4L2_CID_TEST_PATTERN control.

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>