media: dt-bindings: Add bindings for On Semi AR0521 camera sensor

This patch documents DT bindings for the AR0521 camera sensor driver.

[Sakari Ailus: Reworked subject and commit message a little]

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>