board: mntre: imx8mq: Add MNT Reform 2 board support

The MNT Reform 2 is a modular DIY laptop. In its initial version it
is based on the BoundaryDevices i.MX8MQ SoM. Some parts have been
lifted from BoundaryDevices official U-Boot downstream project.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Tested-by: Vagrant Cascadian <vagrant@debian.org>
Tested-by: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Stefano Babic <sbabic@denx.de>