ramips: add support for MeiG SLT866 4G CPE

ramips: add support for MeiG SLT866 4G CPE

Hardware:
 - SoC: Mediatek MT7621 (MT7621AT)
 - Flash: 32 MiB SPI-NOR (Macronix MX25L25635E)
 - RAM: 128 MiB
 - Ethernet: Built-in, 2 x 1GbE
 - 3G/4G Modem: MEIG SLM828 (currently only supported with ModemManager)
 - SLIC: Si32185 (unsupported)
 - Power: 12V via barrel connector
 - Wifi 2.4GHz: Mediatek MT7603BE 802.11b/g/b
 - Wifi 5GHz: Mediatek MT7613BE 802.11ac/n/a
 - LEDs: 8x (7 controllable)
 - Buttons: 2x (RESET, WPS)

Installing OpenWrt:
 - sysupgrade image is compatible with vendor firmware.

Recovery:
 - Connect to any of the Ethernet ports, configure local IP:
   10.10.10.3/24 (or 192.168.10.19/24, depending on OEM)
 - Provide firmware file named 'mt7621.img' on TFTP server.
 - Hold down both, RESET and WPS, then power on the board.
 - Watch network traffic using tcpdump or wireshark in realtime to
   observe progress of device requesting firmware. Once download has
   completed, release both buttons and wait until firmware comes up.

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