ARM: dts: am33xx: Add missing #phy-cells to ti,am335x-usb-phy

"ti,am335x-usb-phy" is using the phy binding, but is missing #phy-cells
property. Fixes the following warning in TI 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: linux-omap@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>