mfd: exynos-lpass: Remove pad retention control

Pad retention should be controlled from pin control driver, so remove it
from Exynos LPASS driver. After this change, no more access to PMU regmap
is needed, so remove also the code for handling PMU regmap.

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