dt-bindings: net: Add bindings for IXP4xx V.35 WAN HSS

This adds device tree bindings for the IXP4xx V.35 WAN high
speed serial (HSS) link.

An example is added to the NPE example where the HSS appears
as a child.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>