media: dt-bindings: tvp5150: cleanup bindings stlye

Use underlines to highlight optional and required properties. This is
quite common for all bindings. Align descriptions and start sentence
with uppercase letter. Also reword the usage of the required
endpoint properties for the output port in case BT.656 should be used.

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>