media: dt-bindings: connector: analog: add sdtv standards property

Some connectors no matter if in- or output supports only a limited
range of sdtv standards. It doesn't matter if the hardware behind that
connector supports more than the listed formats since the users are
restriced by a label e.g. to plug only a camera into this connector
which uses the PAL format.

This patch adds the capability to describe such limitation within the
firmware. There are no format restrictions if the property isn't
present, so it's completely backward compatible.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>