ARM: dts: exynos: Use phandle to get parent node in exynos5250-snow

The i2c-arbitrator node in exynos5250-snow-common used absolute path to
reference other node (the i2c parent). Use phandle instead, because the
depth of the other node may be changed (like moving it under 'soc'
node).

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>