ARM: dts: add sysreg phandle to i2c device nodes for exynos

This patch adds syscon based phandle to i2c device nodes of exynos5250
and exynos5420. These phandles will be used to save restore i2c sysreg
configuration register during s2r from i2c driver.

CC: Rob Herring <robh+dt@kernel.org>
CC: Randy Dunlap <rdunlap@infradead.org>
CC: Russell King <linux@arm.linux.org.uk>
CC: devicetree@vger.kernel.org
CC: linux-doc@vger.kernel.org
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>