arm64: dts: imx8mp-sr-som: build dtbs with symbols for overlay support

arm64: dts: imx8mp-sr-som: build dtbs with symbols for overlay support

Build all dtbs based on SolidRun i.MX8MP SoM with symbols (adding -@ to
dtc flags) to allow applying of device-tree overlays e.g. by the
bootloader.

The SoM has a basler camera connector that can be configured for a
particular camera by device-tree overlay. By extension all boards based
on this SoM have this connector and should support device-tree overlays.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Josua Mayer <josua@solid-run.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>