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.

am335x: add BeagleBone Blue support

BeagleBone Blue is a BeagleBone variant aimed at robotics applications.
Device Tree generated from Linux 4.20.17