ARM: UniPhier: switch to xHCI for PH1-Pro4

PH1-Pro4 includes both EHCI and xHCI IP cores.
Unfortunately, U-Boot cannot enable EHCI and xHCI support
simultaneously.  Some users may wish Super-Speed connection.
Disable CONFIG_USB_EHCI_HCD and enable CONFIG_USB_XHCI_HCD.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>