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