dt-bindings: usb: add #phy-cells to usb-nop-xceiv

Consumers of usb-nop-xceiv use the phy binding, but #phy-cells is missing
from the binding. This is probably because this binding predates the
common phy binding. So add #phy-cells as a required property. This should
not break any users as missing should be treated as 0 cells.

Signed-off-by: Rob Herring <robh@kernel.org>