arm64: dts: imx8mn-var-som-symphony: Add Variscite Symphony board with VAR-SOM-MX8MN

Add a basic DTS for Variscite Symphony evaluation kit with VAR-SOM-MX8MN
(i.MX 8M Nano) System on Module.  This brings up the board with basic
functionalities although still few issues remain (e.g. I2C3 and USB OTG
port, although it might not be the problem of DTS).

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