dt-bindings: usb: ehci: Fix example warnings

The example of the EHCI binding generates a bunch of warnings now that the
examples are validated too.

Most notably, phy-names isn't used at all, and the node name should be USB.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>