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cpu_idx is used as mix of `unsigned int` and `signed int` in code
with typecasting at some places. This change is to unify the
cpu_idx as `unsigned int` as underlying API;s `plat_my_core_pos`
returns `unsigned int`

It was discovered via coverity issue CID 354715
Signed-off-by: Deepika Bhavnani <deepika.bhavnani@arm.com>
Change-Id: I4f0adb0c596ff1177210c5fe803bff853f2e54ce