media: dt-bindings: aspeed-video: Add missing memory-region property

Missed documenting this property in the initial commit.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>