arm: implement generic PSCI reset call for armv8

Add a psci_system_reset() which calls the SYSTEM_RESET function of
PSCI 0.2 and can be used by boards that support it to implement
reset_cpu().

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>