ramips: rt305x: add kernel support for the Planex MZK-W300NH2 Router

ramips: rt305x: add kernel support for the Planex MZK-W300NH2 Router

This is a Japanese market router, for details check
http://www.planex.co.jp/product/router/mzk-w300nh2/. It has only 16MB
RAM, but base OpenWRT install worked fine for days without issues.
Even an image with built-in Luci worked, but don't expect smooth
experience, due to the lack of RAM. Wifi was not extensively tested,
but seems it is working. GPIO LEDs & buttons, factory flash image and
switch config are confirmed working. Tested against revision 34882 +
this patch.

[juhosg: move user-space support and image generation changes into
separate patches]

Signed-off-by: Samir Ibradžić <sibradzic@gmail.com>
Patchwork: http://patchwork.openwrt.org/patch/3077/
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
SVN-Revision: 34925