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 <megous@megous.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190806155744.10263-3-megous@megous.com