ARM: dts: omap: 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 OMAP dts files:

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

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: "Benoît Cousson" <bcousson@baylibre.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Enric Balletbo i Serra <eballetbo@gmail.com>
Cc: Javier Martinez Canillas <javier@dowhile0.org>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>