dt-bindings: panel-simple: Add compatible for GiantPlus GPM940B0

Add a compatible string for the GiantPlus GPM740B0 3" QVGA TFT LCD
panel, and remove the old giantplus,gpm740b0.txt documentation which is
now obsolete.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200109003000.119516-1-paul@crapouillou.net