dt-bindings: usb: exynos-usb: Add missing required VDD properties

Since commit bd8ce544ec35 ("usb: dwc3: exynos: Make provision for vdd
regulators") vdd33-supply and vdd10-supply are required so document them
in bindings.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/usb/exynos-usb.txt

@@ -92,6 +92,8 @@ Required properties:
 	   parent's address space
  - clocks: Clock IDs array as required by the controller.
  - clock-names: names of clocks correseponding to IDs in the clock property
+ - vdd10-supply: 1.0V powr supply
+ - vdd33-supply: 3.0V/3.3V power supply
 
 Sub-nodes:
 The dwc3 core should be added as subnode to Exynos dwc3 glue.
@@ -107,6 +109,8 @@ Example:
 		#address-cells = <1>;
 		#size-cells = <1>;
 		ranges;
+		vdd10-supply = <&ldo11_reg>;
+		vdd33-supply = <&ldo9_reg>;
 
 		dwc3 {
 			compatible = "synopsys,dwc3";