mfd: exynos-lpass: Add support for clocks

Exynos LPASS requires some clocks to be enabled to make any access to its
registers. This patch adds code for handling such clocks. For current set
of registers it is enough to keep sfr0_ctrl clock enabled. Till now it
worked only because those clocks were enabled by bootloader and driver
probe() happened before they were disabled by clock core because of lack
of users. Handling those clocks is also needed to make it possible to
enable support for audio power domain.

This patch requires adding sfr0_ctrl clock to device tree.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-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>