arm64: dts: imx8mm-var-som: Add Variscite VAR-SOM-MX8MM System on Module

Add DTSI of Variscite VAR-SOM-MX8MM System on Module in a basic version,
delivered with Variscite Symphony Evaluation kit.  This version comes
with:
 - 2 GB of RAM,
 - 16 GB eMMC,
 - Gigabit Ethernet PHY,
 - 802.11 ac/a/b/g/n WiFi with 4.2 Bluetooth (Cypress CYW43353),
 - 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>