ipq806x: add support for NEC Platforms Aterm WG2600HP3

ipq806x: add support for NEC Platforms Aterm WG2600HP3

NEC Platforms Aterm WG2600HP3 is a dual-band router based on Qualcomm IPQ8062.

Specification
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- SoC: Qualcomm IPQ8062
- RAM: 512MiB
- Flash memory: SPI-NOR 32MiB (Cypress S25FL256S)
- Wi-Fi: Qualcomm QCA9984 (2.4GHz, 1ch - 13ch)
- Wi-Fi: Qualcomm QCA9984 (5GHz, 36ch - 64ch, 100ch - 140ch)
- Ethernet: 4x 100/1000 Mbps (1x WAN, 4x LAN)
- LED: 6x green LED, 6x red LED
- Input: 2x tactile switch, 1x SP3T slide switch
- Serial console: 115200bps, through-hole J3
  - [  ] [GND] [  ] [TX] [RX] ----> DC jack
- Power: DC 12V 1.5A

This device does not support VHT160 and VHT80+80.
Custom BDFs are required to limit VHT capabilities.

Flash instructions
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1. Setup TFTP server (IP address: 192.168.1.2)
2. Put initramfs image into TFTP server directory
3. Connect WG2600HP3 lan port and computer that runs TFTP server
4. Connect to the serial console
5. Interrupt booting by Esc key (password: chiron)
6. Execute the following commands
    # setenv bootcmd "nboot 0x44000000 1 0x860000"
    # saveenv
    # setenv ipaddr 192.168.1.1
    # setenv serverip 192.168.1.2
    # tftpboot 0x44000000 openwrt-ipq806x-generic-nec_wg2600hp3-initramfs-uImage
7. After booting OpenWrt initramfs image, backup SPI-NOR flash memory
8. Erase firmware partition
    # mtd erase firmware
9. Run sysupgrade

Signed-off-by: Yanase Yuki <dev@zpc.sakura.ne.jp>