ARM: EXYNOS: add generic function to calculate cpu number

The address of cpu power registers in pmu is based on cpu number
offsets. This function calculate the same. This is essentially
required in case of multi-cluster SoC's e.g Exynos5420.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>