drm/panel: simple: Add GiantPlus GPM940B0 panel support

The GiantPlus GPM940B0 is a simple 3.0" 320x240 24-bit TFT panel.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605222247.25657-3-paul@crapouillou.net