media: dt-bindings: adv7180: Document 'adi,force-bt656-4'

The "adv,force-bt656-4" property causes several dt-schema warnings
because 'adv' is not a valid vendor prefix defined in
vendor-prefixes.yaml:

'adv,force-bt656-4' does not match any of the regexes: '^#.*'...

The correct vendor prefix for Analog Devices is 'adi', so change
the property name to 'adi,force-bt656-4'.

Keep the old property for old DTB compatibility and mark it as deprecated.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>