dt-bindings: display: add EDT ET035012DM6 display description

Add support for the ET035012DM6 3.5" QVGA TFT LCD panel. The datasheet
can be retrieved online:

https://www.glynshop.com/erp/owweb/Daten/DSS/EDT/Products/

 \
Specifications/Active%20Displays/ET035012DM6.pdf

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416101630.3482-1-m.felsch@pengutronix.de