dt-bindings: Add LG lp120up1 panel bindings

Add documentation for lp120up1 panel
Signed-off-by: NJitao Shi <jitao.shi@mediatek.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NThierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/display/panel/lg,lp120up1.txt

+LG 12.0" (1920x1280 pixels) TFT LCD panel
+
+Required properties:
+- compatible: should be "lg,lp120up1"
+
+This binding is compatible with the simple-panel binding, which is specified
+in simple-panel.txt in this directory.