ramips: Add support for ComFast CF-E390AX

ramips: Add support for ComFast CF-E390AX

Add support for ComFast CF-E390AX. It is a 802.11 wifi6 cieling AP, based on MediaTek MT7261AT.

Specifications:
SoC: MediaTek MT7621AT
RAM: 128 MiB
Flash: 16 MiB NOR (Macronix mx25l12805d)

Wireless: MT7915E (2.4G) 802.11ax/b/g/n MT7915E (5G) 802.11ac/ax/n
Ethernet: 2 x 1Gbs
Button: 1 x "Reset" button
LED: 1x Blue LED + 1x Red LED + 1x green LED
Power: PoE

Manufacturer Page:
http://en.comfast.com.cn/index.php?m=content&c=index&a=show&catid=84&id=75

Flash Layout:
0x000000000000-0x000000030000 : "bootloader"
0x000000030000-0x000000040000 : "config"
0x000000050000-0x000000060000 : "factory"
0x000000090000-0x000001000000 : "firmware"

First install:
1. Set device into http firmware fail safe upload mode by pressing the reset button for 10 seconds while powering
   it on. Once the LED stops flashing, safe mode will be running.
2. Set PC IP address to 192.168.1.2
3. Browse to 192.168.1.1 and upload the factory image using the web interface.

Signed-off-by: Usama Nassir <usama.nassir@gmail.com>
(cherry picked from commit f24c9b9d863c2635e472e83028d28cc8d0a7c7c6)