ARM: EXYNOS: Consolidate selection of PM_GENERIC_DOMAINS for Exynos4

Instead of repeating "select PM_GENERIC_DOMAINS" for all Exynos4
variants add relevant entry in the Kconfig section common to the
SoC series.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>