src/arch/arm64: Convert to SPDX license header

src/arch/arm64: Convert to SPDX license header

This also drops individual copyright notices, all mentioned authors in
that part of the tree are already listed in AUTHORS.

Change-Id: Ic5eddc961d015328e5a90994b7963e7af83cddd3
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39279
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>