dt-bindings: display: Add support for PDA 91-00156-A0 panel

PDA 91-00156-A0 5.0 is a 5.0" WVGA TFT LCD panel. This panel with
backlight is found in PDA 5" LCD screen (TM5000 series or AC320005-5).
Adding device tree bindings for this panel.
Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>
[eugen.hristev@microchip.com]: specified backlight and supply bindings
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1547458584-29548-3-git-send-email-eugen.hristev@microchip.com

Documentation/devicetree/bindings/display/panel/pda,91-00156-a0.txt

+PDA 91-00156-A0 5.0" WVGA TFT LCD panel
+
+Required properties:
+- compatible: should be "pda,91-00156-a0"
+- power-supply: this panel requires a single power supply. A phandle to a
+regulator needs to be specified here. Compatible with panel-common binding which
+is specified in the panel-common.txt in this directory.
+- backlight: this panel's backlight is controlled by an external backlight
+controller. A phandle to this controller needs to be specified here.
+Compatible with panel-common binding which is specified in the panel-common.txt
+in this directory.
+
+This binding is compatible with the simple-panel binding, which is specified
+in simple-panel.txt in this directory.