mfd: exynos-lpass: Add runtime PM support

Convert exisitng lpass-suspend/resume callbacks into runtime PM callbacks.
This way Exynos LPASS driver will be ready for use with power domains
enabled. LPASS will be runtime resumed/suspended as a result of its child
devices runtime PM transitions.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>