ARM: dts: Fix definition of MSHC device tree nodes for exynos4x12

All SoCs from Exynos4x12 series contain the MSHC block, so its node can
be located in exynos4x12.dtsi. In addition, missing clock specifiers
are added, generic SoC attributes are moved from board dts files
to common dtsi file of SoC family and the node is renamed to a more
generic name to follow node naming recommendations.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>