ar71xx: add support for Teltonika RUT900

ar71xx: add support for Teltonika RUT900

Teltonika RUT900 is an industrial 3G router based on Atheros AR9344.

There are available 3 other models in RUT9xx series: RUT905, RUT950 and
RUT955, which differ in availability of additional I/O ports, built-in
GSM modem type, GPS antenna and other features. FCC ID of the RUT950
model (LTE module built-in): 2AET4-RUT950.

This patch adds support for the RUT900 model only but can be easily
extended to cover whole series. Also, as there are several different
3/4G modules (Huawei, Quectel, Telit) used in whole series, packages
required for WWAN support are not included by default. It is up to the
user to install required software for built-in modem.

Specification:

- 550/400/200 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 4x 10/100 Mbps Ethernet, with passive PoE support on LAN1
- 2T2R 2,4 GHz (AR9344), with ext. PA (MGA-22103) and LNA
- built-in 3G module (example: Telit HE910-D)
- 2x miniSIM slot
- 2x RP-SMA/F (Wi-Fi), 2x SMA/F (3G)
- PCA9539 16-bit GPIO I2C expander
- 12x LED (4 are driven by AR9344, 7 by PCA9539)
- 1x button (reset)
- DC jack for main power input (9-30 V)
- UART available on PCB edge connector

Serial console pinout:

- RX: pin1 (square) on top side of the main PCB (AR9344 is on top)
- TX: pin1 (square) on bottom side

Flash instruction:

Vendor firmware is based on OpenWrt CC release. Use the "factory" image
directly in GUI (make sure to uncheck "keep settings") or in U-Boot web
based recovery. To avoid any problems, make sure to first update vendor
firmware to latest version - "factory" image was successfully tested on
device running "RUT9XX_R_00.03.960" firmware and U-Boot "3.0.1".

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>