arm64: dts: imx8mp: add imx8mp-venice-gw74xx-rpidsi overlay for display

Add support for the following Raspberry Pi displays:
 - DFROBOT DRF0678 7in 800x480 TFT DSI capacitive touch
 - DFROBOT DRF0550 5in 800x480 TFT DSI capacitive touch

Both have the following hardware:
 - FocalTech FT5406 10pt touch controller (with no interrupt)
 - Powertip PH800480T013-IDF02 compatible panel
 - Toshiba TC358762 compatible DSI to DBI bridge
 - ATTINY based regulator used for backlight controller and panel enable

Support is added via a device-tree overlay. The touch controller is not
yet supported as polling mode is needed.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>