Unify type of "cpu_idx" across PSCI module.

NOTE for platform integrators:
   API `plat_psci_stat_get_residency()` third argument
   `last_cpu_idx` is changed from "signed int" to the
   "unsigned int" type.

Issue / Trouble points
1. cpu_idx is used as mix of `unsigned int` and `signed int` in code
with typecasting at some places leading to coverity issues.

2. Underlying platform API's return cpu_idx as `unsigned int`
and comparison is performed with platform specific defines
`PLAFORM_xxx` which is not consistent

Misra Rule 10.4:
The value of a complex expression of integer type may only be cast to
a type that is narrower and of the same signedness as the underlying
type of the expression.

Based on above points, cpu_idx is kept as `unsigned int` to match
the API's and low-level functions and platform defines are updated
where ever required

Signed-off-by: Deepika Bhavnani <deepika.bhavnani@arm.com>
Change-Id: Ib26fd16e420c35527204b126b9b91e8babcc3a5c
diff --git a/docs/getting_started/porting-guide.rst b/docs/getting_started/porting-guide.rst
index 17fd546..bb14717 100644
--- a/docs/getting_started/porting-guide.rst
+++ b/docs/getting_started/porting-guide.rst
@@ -1999,7 +1999,7 @@
 
 ::
 
-    Argument : unsigned int, const psci_power_state_t *, int
+    Argument : unsigned int, const psci_power_state_t *, unsigned int
     Return   : u_register_t
 
 This is an optional interface that is is invoked after resuming from a low power