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.

CMake: Generate device headers from DTS for spike

The DTS compilation was arm platforms only. Moving it to the top level
config file, making it available to RISCV platforms. The generated files
are almost identical with minor differences. A new argument(--arch) is
added to the hardware_gen.py for the differences.