qualcommax: ipq60xx: add GL.iNet GL-AX1800/AXT1800 support

qualcommax: ipq60xx: add GL.iNet GL-AX1800/AXT1800 support

Specifications:
  SoC:     Qualcomm IPQ6000
  RAM:     512 MiB
  Flash:   128 MiB NAND
  ETH:     Qualcomm QCA8075
  WLAN1:   2.4GHz 802.11b/g/n/ax 2x2
  WLAN2:   5GHz 802.11a/n/ac/ax 2x2
  Button:  Reset, Switch
  USB:     1x 3.0
  SD slot: 1 in GL-AXT1800

Install via stock firmware:
  Upload factory.bin in stock firmware's upgrade page,
  do not preserve settings.

Install via uboot web failsafe:
  Push the reset button for 5 seconds, then use broswer
  to access http://192.168.1.1, and upload factory.ubi.

Afterwards upgrade can use sysupgrade image.

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Link: https://github.com/openwrt/openwrt/pull/14950
Signed-off-by: Robert Marko <robimarko@gmail.com>