net: ethernet: ixp4xx: Add DT bindings

This adds device tree bindings for the IXP4xx ethernet
controller with optional MDIO bridge.

Cc: Zoltan HERPAI <wigyori@uid0.hu>
Cc: Raylynn Knight <rayknight@me.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>