arm64: dts: imx8mn-evk: Add HDMI support

imx8mn-evk has a MIPI DSI port that can be used with a ADV7535 MIPI
DSI to HDMI bridge.

Add support for it.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>