ipq806x: add support for NEC Aterm WG2600HP

ipq806x: add support for NEC Aterm WG2600HP

NEC Aterm WG2600HP is a 2.4/5 GHz band 11ac router, based on Qualcomm
IPQ8064.

Specification:

- IPQ8064 (384 - 1,400 MHz)
- 512 MB of RAM
- 32 MB of Flash (SPI)
- 4T4R 2.4/5 GHz
- 5x 10/100/1000 Mbps Ethernet
- 12x LEDs, 4x keys
- 1x USB 3.0 Type-A
- UART header on PCB
  - RX, TX, NC, GND, Vcc from power connector side
  - baudrate: 115200 bps

Flash instruction using initramfs image:
1. Connect serial cable to UART header
2. Connect power cable and turn on the router
3. When the "Press the [f] key and hit [enter] to enter failsafe mode"
message is displayed on the console, press the "f" key and Enter key
sequentially to enter the failsafe mode
4. create fw_env.config file with following contents on failsafe mode:
  /dev/mtd9 0x0 0x10000 0x10000
5. Execute following commands to add and change the environment
variables of U-Boot
  fw_setenv ipaddr "192.168.0.1"
  fw_setenv serverip "192.168.0.2"
  fw_setenv autostart "yes"
  fw_setenv bootcmd "tftpboot 0x44000000 wg2600hp-initramfs.bin;
  bootipq"
6. Set the IP address of the computer to 192.168.0.2, connect to the LAN
port of WG2600HP, and start the TFTP server on the computer
7. Rename OpenWrt initramfs image for WG2600HP to
"wg2600hp-initramfs.bin" and place it in the TFTP directory
8. Remove power cable from WG2600HP, reconnect it and restart WG2600HP
9. WG2600HP downloads initramfs image from TFTP server on the computer,
loads it and boot with initramfs image
10. On the initramfs image, execute "mtd erase firmware" to erase stock
firmware and execute sysupgrade with the sysupgrade image
11. Wait ~180 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>