fix(psci): potential array overflow with cpu on

fix(psci): potential array overflow with cpu on

Fix coverity finding in psci_cpu_on, in which target_idx is directly
assigned the return value from plat_core_pos_by_mpidr. If the latter
returns a negative or large positive value, it can trigger an out of
bounds overflow for the psci_cpu_pd_nodes array.

>>>>    CID 382009:    (OVERRUN)
>>>>    Overrunning callee's array of size 8 by passing argument "target_idx" (which evaluates to 4294967295) in call to "psci_spin_lock_cpu".
> 80         psci_spin_lock_cpu(target_idx);

>>>>    CID 382009:    (OVERRUN)
>>>>    Overrunning callee's array of size 8 by passing argument "target_idx" (which evaluates to 4294967295) in call to "psci_spin_unlock_cpu".
> 160         psci_spin_unlock_cpu(target_idx);

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ibc46934e9ca7fdcaeebd010e5c6954dcf2dcf8c7