dt-bindings: display: Add GiantPlus GPM940B0 panel documentation

The GPM940B0 is a 3.0" 320x240 24-bit TFT LCD panel.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605222247.25657-1-paul@crapouillou.net

Documentation/devicetree/bindings/display/panel/giantplus,gpm940b0.txt

+GiantPlus 3.0" (320x240 pixels) 24-bit TFT LCD panel
+
+Required properties:
+- compatible: should be "giantplus,gpm940b0"
+- power-supply: as specified in the base binding
+
+Optional properties:
+- backlight: as specified in the base binding
+- enable-gpios: as specified in the base binding
+
+This binding is compatible with the simple-panel binding, which is specified
+in simple-panel.txt in this directory.