ARM: mvebu: Add USB nop xceiv support in mvebu_v7_defconfig

The Armada 38x based boards use this "PHY" with their xhci usb
controller. It would be also used for Armada 39x and Armada 375 base
boars which also had xhci controller.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>