arm64: dts: mnt-reform2: add internal display support

arm64: dts: mnt-reform2: add internal display support

This adds support for the internal display of the Reform2 Laptop, which
is connected to the i.MX8MQ via a MIPI-DSI->eDP bridge chip. Clocking
is derived from a system PLL, which provides quite good rate matching
for the single supported display mode and keeps the video PLL free for
usage with the external display, which isn't supported yet.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>