ARM: dts: Add pmu sysreg node to exynos5250 and exynos5420

This patch adds pmusysreg node to exynos5250 and exynos5420 dtsi files to
handle PMU register accesses in a centralized way using syscon driver

Signed-off-by: Leela Krishna Amudala <l.krishna@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>