ar71xx: add kernel support for the TL-WDR3500 board

ar71xx: add kernel support for the TL-WDR3500 board

WDR3500 is similar to WDR3600 except it doesn't have gigabit ethernet,
and has only 1 USB port.

Pending issues:

  * Leds are not working at all (except power and wlan_5g)
  * LAN switch ethernet ports are reversed with respect to case label.

  [Label] -> soft device
  [LAN1] -> eth0.4
  [LAN2] -> eth0.3
  [LAN3] -> eth0.2
  [LAN4] -> eth0.1

Based on http://patchwork.openwrt.org/patch/3208/

Thanks-to: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Gui Iribarren <gui@altermundi.net>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35423 3c298f89-4303-0410-b956-a3cf2f4a3e73