ARM: dts: ixp4xx: Add devicetree for Gateway 7001

This adds a device tree for the Gateway 7001 AP, based on
Intel IXP422.

Cc: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>