There seems to be a problem where SCP can't turn CPU0 off. Instead of
returning PSCI_E_DENIED or crashing make CPU0 wait in a WFE loop. This
way all CPUs have a consistent behaviour from the point of view of the
caller.

Change-Id: I5c8c266ca3b69c9e7a4f5ae70afeea5dd36a0825
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>