ath79: add support for Teltonika RUT955

ath79: add support for Teltonika RUT955

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)
- built-in 4G/3G module (example: Quectel EC-25EU)
- internal microSD slot (spi-mmc, buggy and disabled for now)
- RS232 on D-Sub9 port (Cypress ACM via USB, /dev/ttyACM0)
- RS422/RS485 (AR934x high speed UART, /dev/ttyATH1)
- analog 0-24V input (MCP3221)
- various digital inputs and outputs incl. a relay
- 11x LED (4 are driven by AR9344, 7 by 74HC595)
- 2x miniSIM slot (can be swapped via GPIO)
- 2x RP-SMA/F (Wi-Fi), 3x SMA/F (2x WWAN, GPS)
- 1x button (reset)
- DC jack for main power input (9-30 V)
- debugging UART available on PCB edge connector

Serial console (/dev/ttyS0) 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_00.06.051" firmware and U-Boot "3.0.2".

Signed-off-by: Daniel Golle <daniel@makrotopia.org>