ARM: EXYNOS: Remove redundant code from regs-pmu.h

Commit 6ec4f8d0 ("ARM: EXYNOS: add generic function to calculate
cpu number") introduced exynos_pmu_cpunr to be used by multi-cluster SoCs
e.g. Exynos5420, but it's no more used in the codebase and hence removing
this part of code.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>