PSCI: Fix MISRA defects in common and setup code

MISRA C-2012 Rules 10.1, 10.3, 17.8 and 20.7.

Change-Id: I3980bd2a1d845559af4bbe2887a0250d0506a064
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>