dt-bindings: display: sii902x: Add supply bindings

The sii902x chip family requires IO and core voltages to reach the
correct voltage before chip initialization. Add binding for describing
the two supplies.

Signed-off-by: Alexandru Gagniuc <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/bridge/sii902x.txt

···
 	- interrupts: describe the interrupt line used to inform the host
 	  about hotplug events.
 	- reset-gpios: OF device-tree gpio specification for RST_N pin.
+	- iovcc-supply: I/O Supply Voltage (1.8V or 3.3V)
+	- cvcc12-supply: Digital Core Supply Voltage (1.2V)
 
 	HDMI audio properties:
 	- #sound-dai-cells: <0> or <1>. <0> if only i2s or spdif pin
···
 		compatible = "sil,sii9022";
 		reg = <0x39>;
 		reset-gpios = <&pioA 1 0>;
+		iovcc-supply = <&v3v3_hdmi>;
+		cvcc12-supply = <&v1v2_hdmi>;
 
 		#sound-dai-cells = <0>;
 		sil,i2s-data-lanes = < 0 1 2 >;