ARM: dts: Clean up exynos5250-snow

Use the new style of referencing inherited nodes and use symbolic
names. Reorder one pinctrl node in GPIO order.
Goal is the alignment of all exynos5250 based device trees for
comparison.

Suggested-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Andreas Faerber <afaerber@suse.de>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
[kgene.kim@samsung.com: rebased and squashed]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>