arm64: dts: imx8mn-bsh-smm-s2/pro: Add iMX8MN BSH SMM S2 boards

Introduce BSH SystemMaster (SMM) S2 board family, which consists of:
iMX8MN SMM S2 and iMX8MN SMM S2 PRO boards.

Add support for iMX8MN BSH SMM S2 board:

- 256 MiB DDR3 RAM
- 512 MiB NAND
- Megabit Ethernet PHY
- Wi-Fi 802.11 a/b/g/n/ac with Bluetooth 5.0
- USB-OTG (peripheral mode)

Add support for iMX8MN BSH SMM S2 PRO board:

- 512 MiB DDR3 RAM
- 8 GiB eMMC
- Megabit Ethernet PHY
- Wi-Fi 802.11 a/b/g/n/ac with Bluetooth 5.0
- USB-OTG (peripheral mode)

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>