ar71xx: Add support for Teltonika RUT900

Teltonika RUT900 is a Router with LTE dual SIM, WiFi, 4x Ethernet
ports, I/O, RS232, RS485, GPS.

The device ist based on a Atheros AR9344 rev 3,

Specifications:
- 560/450/225 MHz (CPU/DDR/AHB)
- 128 MB of RAM
- 16 MB of FLASH
- Serial Console header on a Card Board edge connector
- 4x 10/100 Mbps Ethernet (3x LAN, 1x WAN)
- 2.4 GHz Wifi
- 2x external, detachable Wifi antennas
- LTE Modem Huawei ME909u-521 (Also other Modem seen)
- 2x LTE antennas
- 1x GPS antenna
- 7x LED, 1x button
- 1x USB Connector
- 1x Serial RS232
- 1x Serial RS485
- 1x MicroSD Card

The GPL sources of the device are available at www.teltonika.lt/gpl/
and are based on OpenWRT Barrier Breaker (14.07)

Running from tftp:

The Router starts into the uboot Webupdater if the Button ist pressed
more than 3 seconds, if no Network cable is attached it starts the
uboot serial console, from there the router loads the firmware image
via tftpboot from 192.168.1.2:firmware.bin (the router has the
192.168.1.1). With bootm the loaded image will be booted.

Signed-off-by: Steffen Weinreich <steve@weinreich.org>