ARM: uniphier: enable DWC3 xHCI driver really

I thought commit d37d3184 ("ARM: uniphier: enable DWC3 xHCI
driver") enabled CONFIG_USB_DWC3_UNIPHIER, but CONFIG_USB_XHCI_DWC3
was missing in uniphier_v7_defconfig.  Re-add.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>