dt-bindings: panel: lvds: Document power-supply property

The power-supply property is used by a vast majority of panels, including
panel-simple. Let's document it as a common property

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0a6a3abcf1a6b7f0e66a81af8a44c5c0566ce06c.1513854122.git-series.maxime.ripard@free-electrons.com

Documentation/devicetree/bindings/display/panel/panel-common.txt

@@ -78,6 +78,16 @@ used for panels that implement compatible control signals.
   while active. Active high reset signals can be supported by inverting the
   GPIO specifier polarity flag.
 
+Power
+-----
+
+- power-supply: display panels require power to be supplied. While several
+  panels need more than one power supply with panel-specific constraints
+  governing the order and timings of the power supplies, in many cases a single
+  power supply is sufficient, either because the panel has a single power rail,
+  or because all its power rails can be driven by the same supply. In that case
+  the power-supply property specifies the supply powering the panel as a phandle
+  to a regulator.
 
 Backlight
 ---------

Documentation/devicetree/bindings/display/panel/panel-lvds.txt

@@ -32,6 +32,7 @@ Optional properties:
 - label: See panel-common.txt.
 - gpios: See panel-common.txt.
 - backlight: See panel-common.txt.
+- power-supply: See panel-common.txt.
 - data-mirror: If set, reverse the bit order described in the data mappings
   below on all data lanes, transmitting bits for slots 6 to 0 instead of
   0 to 6.

Documentation/devicetree/bindings/display/panel/simple-panel.txt

@@ -1,7 +1,7 @@
 Simple display panel
 
 Required properties:
-- power-supply: regulator to provide the supply voltage
+- power-supply: See panel-common.txt
 
 Optional properties:
 - ddc-i2c-bus: phandle of an I2C controller used for DDC EDID probing