media: dt-bindings: video-interfaces: Document 'orientation' property

Add the 'orientation' device property, used to specify the device mounting
position. The property is particularly meaningful for mobile devices
with a well defined usage orientation.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>