soc: ixp4xx: qmgr: Add DT bindings for IXP4xx qmgr

This adds device tree bindings for the Intel IXP4xx AHB
Queue Manager.

Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>