ARM: EXYNOS4: Implement Clock gating for System MMU

This patch includes the implementation of the clock gating
for System MMU. Initially, all System MMUs are not asserted
the system clock. Asserting the system clock to a System MMU
is enabled only when s5p_sysmmu_enable() is called. Likewise,
it is disabled only when s5p_sysmmu_disable() is called.
Therefore, clock gating on System MMUs are still invisible to
the outside of the System MMU driver.

Signed-off-by: KyongHo Cho <pullip.cho@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>