dt-bindings: display: hdmi-connector: Support DDC bus enable

Some Allwinner SoC using boards (Orange Pi 3 for example) need to enable
on-board voltage shifting logic for the DDC bus using a gpio to be able
to access DDC bus. Use ddc-en-gpios property on the hdmi-connector to
model this.

Add binding documentation for optional ddc-en-gpios property.

Signed-off-by: Ondrej Jirman <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/connector/hdmi-connector.txt

···
 - label: a symbolic name for the connector
 - hpd-gpios: HPD GPIO number
 - ddc-i2c-bus: phandle link to the I2C controller used for DDC EDID probing
+- ddc-en-gpios: signal to enable DDC bus
 
 Required nodes:
 - Video port for HDMI input