ARM: dts: Clean up exynos5250-smdk5250

Use the new style for referencing inherited nodes and use symbolic
names. Goal is the alignment of all exynos5250 based device trees
for comparison.

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>