soc: samsung: pmu: Provide global function to get PMU regmap

PMU is something like a SoC wide service, so add a helper function to get
PMU regmap. This will be used by other Exynos device drivers. This way it
can be avoided to model this dependency in device tree (as phandles to PMU
node) for almost every device in the SoC.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>