dt-bindings: display: msm/dsi: Fix the PHY regulator supply props

The PHY regulator supply names vary across different PHY versions.
Mention explicitly which PHYs require which supplies.

Cc: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

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

@@ -98,7 +98,11 @@ Required properties:
 - clocks: Phandles to device clocks. See [1] for details on clock bindings.
 - clock-names: the following clocks are required:
   * "iface"
+  For 28nm HPM/LP, 28nm 8960 PHYs:
 - vddio-supply: phandle to vdd-io regulator device node
+  For 20nm PHY:
+- vddio-supply: phandle to vdd-io regulator device node
+- vcca-supply: phandle to vcca regulator device node
 
 Optional properties:
 - qcom,dsi-phy-regulator-ldo-mode: Boolean value indicating if the LDO mode PHY