arm64: dts: imx8mp-var-som-symphony: Add Variscite Symphony board and VAR-SOM-MX8MP SoM

Adds the DTSI file for the Variscite VAR-SOM-MX8MP System on Module which
is delivered with the Variscite Symphony Evaluation Kit.

Initial support includes:
- Serial console
- eMMC
- SD card

Signed-off-by: Tarang Raval <tarang.raval@siliconsignals.io>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>