xen: Add licence header to device_tree_defs.h

xen: Add licence header to device_tree_defs.h

This header have been created by moving code from other part of the
project and miss a licence header. The original source code was some
version of GPL or LGPL but we intend to have the public header to be
MIT so they can be included easily in other projects.

Part of device_tree_defs.h were moved from libxl_arm.c which is
LGPL-2.1-only. And part were moved from device_tree.h that is
GPL-2.0-only.

Part of the original code were added by Julien Grall @ Linaro in
commits c3ba52a84dd8 and 405c167f0ec9 and 886f34045bf0. The other part
were added by Ian Campbell @ Citrix, with commit 0c64527e7fc9.

Resolves: xen-project/xen#35
Fixes: 1c898a9fec7e ("xen/arm: move a few DT related defines to public/device_tree_defs.h")
Reported-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com> [Citrix relicensing]
Acked-by: Grant Likely <grant.likely@linaro.org> [Linaro relicensing]
Release-acked-by: Henry Wang <Henry.Wang@arm.com>