The `arm-gic.h` file distributed by the Linux kernel is disjunctively
dual-licensed under the GPL-2.0 or MIT licenses, but the BSD-3-Clause
license has been applied in violation of the requirements of both
licenses. This change ensures the file is correctly licensed under the
terms of the MIT license, and that we comply with it by distributing a
copy of the license text.

Change-Id: Ie90066753a5eb8c0e2fc95ba43e3f5bcbe2fa459
Signed-off-by: Chris Kay <chris.kay@arm.com>