ARM: EXYNOS: Split up exynos5420 SoC specific PMU data

This patch splits up mach-exynos/pmu.c file, and moves exynos5420,
PMU configuration data and functions handing data into exynos5420
SoC specific PMU file mach-exynos/exynos5420-pmu.c.

[tested on Peach-Pi (Exynos5880)]
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>