soc: samsung: exynos-pmu: instantiate clkout driver as MFD

The Exynos clock output (clkout) driver uses same register address space
(Power Management Unit address space) as Exynos PMU driver and same set
of compatibles.  It was modeled as clock provider instantiated with
CLK_OF_DECLARE_DRIVER().

This however brings ordering problems and lack of probe deferral,
therefore clkout driver should be converted to a regular module and
instantiated as a child of PMU driver to be able to use existing
compatibles and address space.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Tested-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20201001165646.32279-2-krzk@kernel.org