imx8mn_var_som: Add support for Variscite VAR-SOM-MX8M-NANO board

Add support for iMX8MN VAR-SOM-MX8M-NANO board. Enables support for:

- 1GiB DDR4 RAM
- 16 GiB eMMC
- SD card
- Gigabit ethernet
- USBOTG1 peripheral - fastboot

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>