ARM: dt: relicense two DT binding IRQ headers

Files in include/dt-bindings/ may be useful to any OS that uses DT, when
building the OS binary itself, not just when building DTB files. Since
some OSs are not GPL, they need non-GPL headers. This change relicenses
two of the useful files so that they can be used under the MIT license
when desired. I wrote these files and NVIDIA legal has approved this
change. Geert also ack'd the change; he fixed some spelling issues in the
comments.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NRob Herring <robh@kernel.org>

include/dt-bindings/interrupt-controller/arm-gic.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /*
  * This header provides constants for the ARM GIC.
  */

include/dt-bindings/interrupt-controller/irq.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /*
  * This header provides constants for most IRQ bindings.
  *