dt-bindings: usb: ehci: Add missing 2nd register region

The 'ibm,usb-ehci-440epx' compatible has a 2nd 'reg' region, but the
schema says there is only 1 region. Fix this.

Cc: linux-usb@vger.kernel.org
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <robh@kernel.org>