drm/panel: simple: Add support for EDT ET035012DM6

Add support for the EDT ET035012DM6 3.5" 320x240 QVGA 24-bit RGB TFT.

The datasheet with all specs can be retrieved online:
https://www.glynshop.com/erp/owweb/Daten/DSS/EDT/Products/

 \
Specifications/Active%20Displays/ET035012DM6.pdf

Signed-off-by: Andreas Pretzsch <apr@cn-eng.de>
[m.felsch@pengutronix.de: adapt commit message]
[m.felsch@pengutronix.de: rm unecessary comments]
[m.felsch@pengutronix.de: correct data_enable polarity]
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416101630.3482-2-m.felsch@pengutronix.de