ramips: add support for Comfast CF-EW84

ramips: add support for Comfast CF-EW84

Add support for the Comfast CF-EW84 based on the MediaTek MT7621
SoC.

Hardware:
  CPU/SoC:      MediaTek MT7621DA
  RAM:          128 MB DDR3
  Flash:        Macronix MX25L12805D 16 MB
  Switch:       MediaTek MT7530
  WiFi 2.4 GHz: MediaTek MT7603E
  WiFi 5 GHz:   MediaTek MT7613B
  Ports:        1x WAN, 2x LAN (10/100/1000 Mbps)
  LEDs:         Power, WAN, LAN1, LAN2, WLAN
  Buttons:      Reset
  UART:         through-hole on PCB
                pinout: RX, TX, GND, 3.3V
                settings: 115200n8

Flash instructions:
1. Connect the bundled PoE injector to power.
2. Connect the PC to the LAN1 or LAN2 port of the device.
3. Configure a static IP on the computer's Ethernet adapter:
     IP:      192.168.1.x (x = 2 to 254)
     Mask:    255.255.255.0
     Gateway: empty
     DNS:     empty
4. Connect a second Ethernet cable to the PoE port of the
   bundled injector.
5. Press and hold the reset button on the device.
6. While holding the reset button, plug the other end of the
   second Ethernet cable into the WAN port.
7. Wait at least 3 seconds, then release the reset button.
8. Browse to http://192.168.1.1: the MediaTek U-Boot System
   Recovery page should appear.
9. Click Upload and select the *-sysupgrade.bin firmware file.
10. After flashing completes, browse to http://192.168.1.1
    again: the LuCI login page should appear.

Factory data:
  2.4 GHz EEPROM:        factory offset 0x0000
  5 GHz EEPROM:          factory offset 0x8000
  LAN MAC address:       factory offset 0xe000
  WAN MAC address:       factory offset 0xe000 (+1)

LED layout:
The Comfast CF-EW84 has five physical LEDs, but only the WLAN
LED is software-controllable through the Linux GPIO-LEDs
subsystem.

Signed-off-by: Vincenzo Suraci <vincenzo.suraci@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/24112
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>