ARM: dts: imx6qdl: make VAR-SOM SoM SoC-agnostic

ARM: dts: imx6qdl: make VAR-SOM SoM SoC-agnostic

Make SoM .dtsi SoC-agnostic by moving SoC include to board level

imx6qdl-var-som.dtsi currently includes imx6q.dtsi, which makes this SoM
description Quad/Dual specific and prevents reuse from i.MX6DL boards.

Changes:
 - Move imx6q.dtsi include from imx6qdl-var-som.dtsi to
imx6q-var-mx6customboard.dts.
 - Remove /dts-v1/; from imx6qdl-var-som.dtsi (dtsi files should not declare
version)

This keeps the SoM .dtsi SoC-agnostic (it already relies on imx6qdl.dtsi for
family-common parts) and allows boards using the DualLite or Solo to include
imx6dl.dtsi instead.

Why this is needed:
To reuse imx6qdl-var-som.dtsi on i.MX6DL board.

No functional changes for imx6q-var-mx6customboard are intended.

Signed-off-by: Stefan Prisacariu <stefan.prisacariu@prevas.dk>
Link: https://lore.kernel.org/all/20250925104942.4148376-1-stefan.prisacariu@prevas.dk/
Signed-off-by: Shawn Guo <shawnguo@kernel.org>