feat(cpus): add a64fx cpu to tf-a

while sbsa maintainers upstream decide whether new cpus types
should be in, add fujitsu a64fx cpu type in advance

Signed-off-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
Change-Id: I521a62f1233f3fe6e92f040edaff2cc60a1bd874