arm64: dts: colibri-imx8x: Prepare for qxp and dx variants

Toradex sells the Colibri iMX8X module in variants with the i.MX 8QXP
and i.MX8DX SoC. Prepare for this by moving majority of stuff from
imx8qxp-colibri.dtsi into imx8x-colibri.dtsi.

Remove DX from the model string.

This commit intends no functional change.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>