Switch to the new device-tree vendor tree

The old vendor tree was never fully merged and doing partial merge isn't
supported with git subtree merge so a new one was created.
Switch the build to use the new DTS from sys/contrib/device-tree
This also bump the DTS used to be in sync with Linux 5.9
While here change the way to get the linux version, simply hardcode
the value in sys/dts/freebsd-compatible.dts and use awk to get that
to put it in the CFLAGS.
As a bonus we now have the bindings docs available
in sys/contrib/device-tree/Bindings/ so no need to link to the Linux repo
or to the vendor tree.