ath79: add support for Teltonika RUT955 H7V3C0

ath79: add support for Teltonika RUT955 H7V3C0

This board was previously supported in ar71xx as 'RUT9XX'. The
difference between that and the other RUT955 board already supported in
ath79 is that instead of the SPI shift registers driving the LEDs and
digital outputs that model got an I2C GPIO expander instead.

To support LEDs during early boot and interrupt-driven digital inputs,
I2C support as well as support for PCA953x has to be built-in and
cannot be kernel modules, hence select those symbols for ath79/generic.

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 PCA9539)
- 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 "3.0.1".

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