ARM: dts: Simplify USB3503 on exynos5250-arndale

There's no need for a simple-bus, place the smsc,usb3503a directly into
the root node. That's what we're going to do on exynos5250-spring.

Reported-by: Tomasz Figa <t.figa@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>