media: dt-bindings: display: add sdtv-standards defines

Add defines which can be included to easily describe the supported
standard tv norms 'sdtv-standards' within the device tree.

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