ARM: dts: add pmu sysreg node to exynos3250

This patch add pmusysreg node for Exynos3250 to access PMU
(Power Management Unit) register in a centralized way using
syscon driver.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>