dt-bindings: display: stm32: correct clock-names in dsi panel example

In the dsi panel example, clock names in the "clock-names"
field have been swapped:
* "pclk" (peripheral clock) is <&rcc 1 CLK_F469_DSI> on stm32f4
* "ref" (dsi phy pll ref clock) is <&clk_hse> on stm32f4
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180122153547.23470-1-philippe.cornu@st.com

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

@@ -98,7 +98,7 @@ Example 2: DSI panel
 			compatible = "st,stm32-dsi";
 			reg = <0x40016c00 0x800>;
 			clocks = <&rcc 1 CLK_F469_DSI>, <&clk_hse>;
-			clock-names = "ref", "pclk";
+			clock-names = "pclk", "ref";
 			resets = <&rcc STM32F4_APB2_RESET(DSI)>;
 			reset-names = "apb";