ARM: dts: add PMU syscon node for exynos4

This patch adds a PMU(Power Management Unit) syscon node. This
should be required for USB Phy syscon regmap I/F.

Cc: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>