ARM: keystone: defconfig: Fix USB configuration

Simply enabling CONFIG_KEYSTONE_USB_PHY doesn't work anymore
as it depends on CONFIG_NOP_USB_XCEIV. We need to enable
that as well.

This fixes USB on Keystone boards from v4.8-rc1 onwards.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>