ARM: dts: ixp4xx: Add devicetree for Linksys WRV54G

This adds a device tree for the Linksys WRV54G also known as
Gemtek GTWX5715. Some enhancements have been folded in from the
OpenWrt patches.

This supports everything in the upstream kernel with placeholders
for the out-of-tree multiphy which exist in OpenWrt.

Cc: phj@phj.hu
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>