fix(psci): tighten psci_power_down_wfi behaviour

fix(psci): tighten psci_power_down_wfi behaviour

A processing element should never return from a wfi, however, due to a
hardware bug, certain CPUs may wake up because of an external event.
This patch tightens the behaviour of the common power down sequence, it
ensures the routine never returns by entering a wfi loop at its end. It
aligns with the behaviour of the platform implementations.

Change-Id: I36d8b0c64eccb71035bf164b4cd658d66ed7beb4
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>