dt-bindings: display: stm32: add supply property to DSI controller

This patch adds documentation of a new property phy-dsi-supply to the
STM32 DSI controller.
Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1557826556-10079-2-git-send-email-yannick.fertre@st.com

Documentation/devicetree/bindings/display/st,stm32-ltdc.txt

@@ -40,6 +40,8 @@ Mandatory nodes specific to STM32 DSI:
 - panel or bridge node: A node containing the panel or bridge description as
   documented in [6].
   - port: panel or bridge port node, connected to the DSI output port (port@1).
+Optional properties:
+- phy-dsi-supply: phandle of the regulator that provides the supply voltage.
 
 Note: You can find more documentation in the following references
 [1] Documentation/devicetree/bindings/clock/clock-bindings.txt
@@ -101,6 +103,7 @@ Example 2: DSI panel
 			clock-names = "pclk", "ref";
 			resets = <&rcc STM32F4_APB2_RESET(DSI)>;
 			reset-names = "apb";
+			phy-dsi-supply = <&reg18>;
 
 			ports {
 				#address-cells = <1>;