ath79: add support for Dongwon T&I DW02-412H

ath79: add support for Dongwon T&I DW02-412H

Dongwon T&I DW02-412H is a 2.4/5GHz band 11ac (WiFi-5) router, based on
Qualcomm Atheros QCA9557.

Specifications
--------------

- SoC: Qualcomm Atheros QCA9557-AT4A
- RAM: DDR2 128MB
- Flash: SPI NOR 2MB (Winbond W25Q16DVSSIG / ESMT F25L16PA(2S)) +
         NAND 64/128MB
- WiFi:
  - 2.4GHz: QCA9557 WMAC
  - 5GHz: QCA9882-BR4A
- Ethernet: 5x 10/100/1000Mbps
  - Switch: QCA8337N-AL3C
- USB: 1x USB 2.0
- UART:
  - JP2: 3.3V, TX, RX, GND (3.3V is the square pad) / 115200 8N1

Installation
--------------

1.  Connect a serial interface to UART header and
    interrupt the autostart of kernel.
2.  Transfer the factory image via TFTP and write it to the NAND flash.
3.  Update U-Boot environment variable.
    > tftpboot 0x81000000 <your image>-factory.img
    > nand erase 0x1000000
    > nand write 0x81000000 0x1000000 ${filesize}
    > setenv bootpart 2
    > saveenv

Revert to stock firmware
--------------

1.  Revert to stock U-Boot environment variable.
    > setenv bootpart 1
    > saveenv

MAC addresses as verified by OEM firmware
--------------

   WAN: *:XX (label)
   LAN: *:XX + 1
  2.4G: *:XX + 3
    5G: *:XX + 4

The label MAC address was found in art 0x0.

Credits
--------------

Credit goes to the @manatails who first developed how to port OpenWRT
to this device and had a significant impact on this patch.

And thanks to @adschm and @mans0n for guiding me to revise the code
in many ways.

Signed-off-by: Jihoon Han <rapid_renard@renard.ga>
Reviewed-by: Sungbo Eo <mans0n@gorani.run>
Tested-by: Sungbo Eo <mans0n@gorani.run>