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>