ramips: add support for D-Link DIR-806A B1 router

ramips: add support for D-Link DIR-806A B1 router

General specification:
SoC Type: MediaTek MT7620A (580MHz)
ROM: 8 MB SPI-NOR (MX25L6406E)
RAM: 64 MB DDR (W9751G6KB-25)
Switch: MediaTek MT7530
Ethernet: 5 ports - 5×100MbE (WAN, LAN1-4)
Wireless: 2.4 GHz (MediaTek RT5390): b/g/n
Wireless: 5 GHz (MediaTek MT7610EN): ac/n
Buttons: 2 button (POWER, WPS/RESET)
Bootloader: U-Boot 1.1.3
Power: 12 VDC, 0.5 A

MACs:
| LAN	| [Factory + 0x04] - 2		|
| WLAN 2.4g	| [Factory + 0x04] - 1		|
| WLAN 5g	| [Factory + 0x8004] - 3	|
| WAN	| [Factory + 0x04] - 2		|

OEM easy installation:

1. Use a PC to browse to http://192.168.0.1.
2. Go to the System section and open the Firmware Update section.
3. Under the Local Update at the right, click on the CHOOSE FILE...
4. When a modal window appears, choose the firmware file and click on
 the Open.
5. Next click on the UPDATE FIRMWARE button and upload the firmware image.
Wait for the router to flash and reboot.

OEM installation using the TFTP method (need level converter):

1. Download the latest firmware image.
2. Set up a Tftp server on a PC (e.g. Tftpd32) and place the firmware
 image to the root directory of the server.
3. Power off the router and use a twisted pair cable to connect the PC
 to any of the router's LAN ports.
4. Configure the network adapter of the PC to use IP address 192.168.0.180
 and subnet mask 255.255.255.0.
5. Connect serial port (57600 8N1) and turn on the router.
6. Then interrupt "U-Boot Boot Menu" by hitting 2 key (select "2: Load
 system code then write to Flash via TFTP.").
7. Press Y key when show "Warning!! Erase Linux in Flash then burn new
 one. Are you sure? (Y/N)"
Input device IP (192.168.0.1) ==:192.168.0.1
Input server IP (192.168.0.180) ==:192.168.0.180
Input Linux Kernel filename () ==:firmware_name
The router should download the firmware via TFTP and complete flashing in
 a few minutes.
After flashing is complete, use the PC to browse to http://192.168.1.1 or
 ssh to proceed with the configuration.

Signed-off-by: Alexey Bartenev <41exey@proton.me>
(cherry picked from commit ce998cb6e161db9c9e868fd570da9189f05fde3e)