dt-bindings: msm/dsi: Add ref clock for PHYs

Allow the PHY drivers to get the ref clock from the DT.
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

Documentation/devicetree/bindings/display/msm/dsi.txt

@@ -106,6 +106,7 @@ Required properties:
 - clocks: Phandles to device clocks. See [1] for details on clock bindings.
 - clock-names: the following clocks are required:
   * "iface"
+  * "ref" (only required for new DTS files/entries)
   For 28nm HPM/LP, 28nm 8960 PHYs:
 - vddio-supply: phandle to vdd-io regulator device node
   For 20nm PHY: