review Linux-derived dts file license

These files are derived from the output of the device tree compiler in
the Linux kernel. The licenses of the input files do all have to be
compatible with at least GPL-2.0-only to be part of Linux.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

review third-party license tags

Made third-party license tags more precise.

Move DTS to the kernel

Move DTS to the kernel in preparation for using them to autogenerate
hardware headers.

This includes DTS that we didn't have previously, extracted from the
Linux kernel. Everything except TX2 comes from Linux v4.20, extracted
with the following commands:

checkout https://github.com/torvalds/linux.git v4.20
./update-dts.sh /path/to/linux/checkout

The TX2 dts is identical to the one that was found in seL4_tools.