arm: dts: nspire: Add missing #phy-cells to usb-nop-xceiv

"usb-nop-xceiv" is using the phy binding, but is missing #phy-cells
property. This is probably because the binding was the precursor to the phy
binding.

Fixes the following warning in nspire dts files:

Warning (phys_property): Missing property '#phy-cells' in node ...

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>