dt-bindings: usb: tegra-xudc: Remove extraneous PHYs

The USB device controller on Tegra210 and later supports one USB 2/3
port, so only a single pair of PHYs is needed. Drop any of the extra
PHYs from the bindings.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>