ar71xx: add support for TP-Link TL-MR3020

ar71xx: add support for TP-Link TL-MR3020

This patch adds support for the TP-Link device TL-MR3020 which is very
similiar to the TL-WR703N.
The TL-MR3020 uses 5 LEDs, 1 push button and a switch with 3 positions.
Only four LEDs can be controlled via GPIO. The power LED is connected to
the power supply.
The WPS push button has been mapped to KEY_WPS_BUTTON. The two GPIO signals
of the sliding switch have been mapped to BTN_0 and BTN_1.

I have also setup a wiki page with details here:
http://wiki.openwrt.org/toh/tp-link/tl-mr3020

USB is working and has been tested with USB mass storage and USB UMTS
stick.

Wifi also seems to work.

Ethernet is working too.

Signed-off-by: Christian Cier-Zniewski <c.cier@gmx.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@29651 3c298f89-4303-0410-b956-a3cf2f4a3e73