arm64: dts: imx8mn-var-som: Add Variscite VAR-SOM-MX8MN System on Module

Add DTSI of Variscite VAR-SOM-MX8MN (Nano) System on Module in a basic
version, delivered with Variscite Symphony Evaluation kit.  This version
comes with:
- 1 GB of RAM,
- 16 GB eMMC,
- Gigabit Ethernet PHY,
- 802.11 ac/a/b/g/n WiFi with 4.2 Bluetooth,
- CAN bus,
- Audio codec (not yet configured in DTSI).

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>