PSCI: Fix MISRA defects in ON/OFF/SUSPEND/SYSTEM_OFF

Fix violations of MISRA C-2012 Rules 8.13, 10.1, 10.3, 17.7 and 20.7.

Change-Id: I6f45a1069b742aebf9e1d6a403717b1522083f51
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>