multi_v7_defconfig: make Rockchip usb2-phy built-in

The phy is necessary for the dwc2 controllers driving the usb ports
on all arm32 Rockchip socs. Both the dwc2 as well as usb downstream
drivers (mass-storage as well as usb networking) are already built-in,
so only the phy is missing to allow booting from usb-devices without
to much hassle.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Olof Johansson <olof@lixom.net>