ipq807x: add support for Netgear WAX620

ipq807x: add support for Netgear WAX620

```
Specifications:
* CPU: Qualcomm IPQ8072A, SoC Version: 2.0, Quad core Cortex-A53 1.6896 GHz
* RAM: 1 GiB of DDR4 600 MHz
* Flash: NAND 2x256 MiB (Macronix MX30UF2G18AC)
* 4 RGB LEDs: Power, LAN, 2.4GHz and 5GHz
* UART: Two 4-pin unpopulated headers under the LEDs.
  Use the header closest to LED 4 and 5.
  They are marked with a white stroke.
  TX RX GND, beginning from "4". 115200n8.

Lan:
* One 100/1000/2.5GBASE-T Gigabit Ethernet (QCA8081)

Wlan:
* 4x4 in 2.4GHz: 802.11b/g/n/ax
* 4x4 in 5.0GHz: 802.11a/n/ac/ax
* OFDM and OFDMA
* Bidir and MU-MIMO
* Internal antenna 3.1/4.3 dBi (2.4GHz/5GHz)

Power:
* PoE+ 802.3at/af 25.5W
* DC 12V 2.5A
```

```
Note: The OpenWrt image is setup with DHCP and not a static IP.
1.  Download the OpenWrt initramfs image. Copy the image to a TFTP server
2.  Connect to console on the AP, and connect the LAN port to your LAN
3.  Stop auto boot to get to U-boot shell, interrupt the autoboot process by pressing '0' when prompted
4.  Set active_fw in env
    # setenv active_fw 1
5.  Transfer the initramfs image with TFTP
    # setenv serverip 192.168.1.10 (IP of TFTP server host)
    # setenv ipaddr 192.168.1.1 (IP used by the router for getting the image, must be in the same subnet as the TFTP host)
    # tftpboot openwrt-qualcommax-ipq807x-netgear_wax620-initramfs-uImage.itb
6.  Reboot and load the image
    # bootm
7.  SCP factory image to the AP
    # scp openwrt-qualcommax-ipq807x-netgear_wax620-squashfs-factory.ubi root@192.168.1.1:/tmp/
8.  Connect to device using SSH (use the LAN port)
9.  Flash squashfs-factory.ubi from within the initramfs instance of OpenWRT
    Before you flash, please check your mtd partitions where mtdX is the right mtd rootfs partition.
    # cat /proc/mtd (To check MTD partitions)
    # ubiformat /dev/mtd19 -y -f /tmp/openwrt-qualcommax-ipq807x-netgear_wax620-squashfs-factory.ubi
10. Set active_fw to 0
    # /usr/sbin/fw_setenv active_fw 0
11. Reboot the AP and your done
    # reboot
```

Signed-off-by: Kristian Skramstad <kristian+github@83.no>