arm64: dts: imx8mq: add support for MNT Reform2

This adds a basic devicetree for the MNT Reform2 DIY laptop. Not all
of the board periperals are enabled yet, as some of them still require
kernel patches to work properly. The nodes for those peripherals will
be added as soon as the required patches are upstream.

The following has been tested to work:
- UART console
- SD card
- eMMC
- Gigabit Ethernet
- USB (internal Keyboard, Mouse, external ports)
- M.2 PCIe port

Co-developed-by: Lukas F. Hartmann <lukas@mntre.com>
Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>