ar71xx: add Arduino Yun

ar71xx: add Arduino Yun

This is based on patches from Federico Fissore <f.fissore@arduino.cc>
especially this one:
https://github.com/arduino/openwrt-yun/commit/7e2976fa83fb0d72a955574499178c387a1b3c92

The console is running with 250000 baud which is a non standard baud
rate and needs an extra patch to be applied, I will try to get this
patch upstream or something else which accomplish the same.

Some upstream code looks like there are many different versions of this
SoC, are these only internal versions and all versions on the consumer
market are the same? I saw different GPIO configuration and flash sizes
of 8MB and 16MB?

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
SVN-Revision: 47451