arm64: Break circular dependency on syscall.h

Commit fbf851cd is introducing a
dependency between util.h and time_units.h.

Now, the problem is that time_units.h is from one side included by
arm64/syscall.h (syscall.h -> lib_helpers.h -> cpu.h -> util.h) but on
the other side it is including syscall.h (time_units.h -> syscall.h).

Break the dependency at cpu.h level.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>