ramips: add support for BOLT! Arion

ramips: add support for BOLT! Arion

This device is from now-defunct BOLT! ISP in Indonesia.
The original firmware is based on mediatek SDK running linux 2.6 or 3.x in later revision.

Specifications:

- SoC:      MediaTek MT7621
- Flash:    32 MiB NOR SPI
- RAM:      128 MiB DDR3
- Ethernet: 2x 10/100/1000 Mbps (switched, LAN + WAN)
- WIFI0:    MT7603E 2.4GHz 802.11b/g/n
- WIFI1:    MT7612E 5GHz 802.11ac
- Antennas: 2x internal, non-detachable
- LEDs:     Programmable LEDs: 5 blue LEDs (wlan, tel, sig1-3) and 2 red LEDs (wlan and sig1)
            Non-programmable "Power"  LED
- Buttons:  Reset and WPS

Instalation:
Install from TFTP

Set your PC IP to 10.10.10.3 and gateway to 10.10.10.123
Press "1" when turning on the router, and type the initramfs file name

You also need to solder pin header or cable to J4 or neighboring test points (T19-T21)
Pinouts from top to bottom: GND, TX, RX, VCC (3.3v)
Baudrate: 57600n8

There's also an additional gigabit transformer and RTL8211FD managed by the LTE module on the backside of the PCB.

Signed-off-by: Abdul Aziz Amar <abdulaziz.amar@gmail.com>
(cherry picked from commit 78c3534645958c123aa82cec9926a34eed5dd5dd)