dt-bindings: display: amlogic, meson-dw-hdmi: Add optional HDMI 5V regulator

On reference boards and derivatives, the HDMI Logic is powered by an external
5V regulator.
This regulator was set by the Vendor U-Boot, add optional support for it.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1512561268-29806-3-git-send-email-narmstrong@baylibre.com

Documentation/devicetree/bindings/display/amlogic,meson-dw-hdmi.txt

@@ -48,6 +48,10 @@ Required properties:
   Documentation/devicetree/bindings/reset/reset.txt,
   the reset-names should be "hdmitx_apb", "hdmitx", "hdmitx_phy"
 
+Optional properties:
+- hdmi-supply: Optional phandle to an external 5V regulator to power the HDMI
+  logic, as described in the file ../regulator/regulator.txt
+
 Required nodes:
 
 The connections to the HDMI ports are modeled using the OF graph