exynos: Enable XHCI on exynos5250 boards

exynos: Enable XHCI on exynos5250 boards

Once upon a time u-boot didn't support building with two usb host
controller types, these days it does. Enable XHCI in addition to the
existing EHCI support so user can plug usb devices in all available
ports regardless of the controller type.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>