arm64: dts: imx8mm-var-som: drop unused top-level compatible

The Variscite VAR-SOM-MX8MM System-on-Module cannot be used alone
without motherboard, so drop the top-level compatible field to avoid any
false impression that such usage is possible.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>