media: sun6i: Add A31 compatible

The first device that used that IP was the A31. Add it to our list of
compatibles.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>