ULPI PHYs have a reset signal, and different boards use a different GPIO
for this task. Add a property to device tree to represent this.

I'm not sure if adding this property to the EHCI controller node is
entirely correct; perhaps eventually we should have explicit separate
nodes for the various PHYs. However, we don't have that right now, so this
binding seems like a reasonable choice.

Cc: <devicetree-discuss@lists.ozlabs.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <linux-usb@vger.kernel.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>