dt-bindings: display: bridge: Convert simple-bridge bindings to YAML

The simple-bridge driver supports multiple simple or dumb bridges,
covered by different compatible strings but otherwise identical DT
bindings. Some of those bridges have undocumented bindings, while others
are documented in text form in separate files. Group them all in a
single binding and convert it to YAML.

The psave-gpios property of the adi,adv7123 is dropped, as it isn't
supported by the driver and isn't specified in any DT file upstream.
Support for power saving is available through the enable-gpios property
that should cover all the needs of the ADV7123 (as the device only has a
/PSAVE pin and no enable pin).

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>