[media] dt/bindings: device tree description for AD5820 camera auto-focus coil

Add documentation for ad5820 device tree binding.

Signed-off-by: Pavel Machek <pavel@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>