dt-bindings: display: rockchip: dw-hdmi: Add regulator support

The RK3568 has HDMI_TX_AVDD0V9 and HDMI_TX_AVDD_1V8 supply inputs
needed for the HDMI port. Add the binding for these supplies.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220422072841.2206452-11-s.hauer@pengutronix.de

Documentation/devicetree/bindings/display/rockchip/rockchip,dw-hdmi.yaml

@@ -28,6 +28,17 @@ properties:
   reg-io-width:
     const: 4
 
+  avdd-0v9-supply:
+    description:
+      A 0.9V supply that powers up the SoC internal circuitry. The actual pin name
+      varies between the different SoCs and is usually HDMI_TX_AVDD_0V9 or sometimes
+      HDMI_AVDD_1V0.
+
+  avdd-1v8-supply:
+    description:
+      A 1.8V supply that powers up the SoC internal circuitry. The pin name on the
+      SoC usually is HDMI_TX_AVDD_1V8.
+
   clocks:
     minItems: 2
     items: