dt-bindings: Add doc for the Ingenic JZ47xx LCD controller driver

Add documentation for the devicetree bindings of the LCD controller
present in the JZ47xx family of SoCs from Ingenic.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190603152331.23160-1-paul@crapouillou.net