arm64: dts: imx8mq-nitrogen: add lt8912 MIPI-DSI to HDMI

Add support of the lt8912b in the DTB.
This adds the support of the DB_DSIHD daugther board from
Boundary Devices.

Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>