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