dts: Move vendor-specific dtsi to dedicated folder

Move all the vendor-specific dtsi files that were in dts/common to a
new folder under dts/ designated for vendor-specific files,
since they are not common at all, except for one vendor.

Change MAINTAINERS.yml to reflect the moving of the files.

Update migration guide for this change.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>