ARM: dts: Add sysreg sytem controller node to exynos5250 and exynos5420

This patch adds sysreg-syscon node to exynos5250 and exynos5420 device
tree, to access System Register's registers using syscon driver.

Signed-off-by: Kamil Debski <k.debski@samsung.com>
[gautam.vivek@samsung.com: Split this syreg-syscon dts entry]
[gautam.vivek@samsung.com: added similar syscon entry for exynos5420]
Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
[vikas.sajjan@samsung.com: updated the binding document]
Signed-off-by: Vikas Sajjan <vikas.sajjan@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>