linux/imx8mq-mnt-reform2-hdmi.dts: append 'HDMI' to model name

There is currently no easy way to determine which dtb is currently enabled. By having the wrong /boot mounted, it is possible to run reform-display-config and not have any effect. It's hard to debug this, because it is not obvious which dtb is currently loaded at runtime. With this change, it should be as simple as reading /proc/device-tree/model to figure out which dtb is currently used.