ipq807x: add Redmi AX6

ipq807x: add Redmi AX6

Redmi AX6 is a budget 802.11ax dual-band router/AP

Specifications:
* CPU: Qualcomm IPQ8071A Quad core Cortex-A53 1.4GHz
* RAM: 512MB of DDR3
* Storage: 128MB NAND
* Ethernet: 4x1G RJ45 ports (QCA8075)
* WLAN:
	* 2.4GHz: Qualcomm QCN5024 2x2 802.11b/g/n/ax 574 Mbps PHY rate
	* 5GHz: Qualcomm QCN5054 4x4@80MHz or 2x2@160MHz 802.11a/b/g/n/ac/ax 2402 PHY rate
* LEDs:
	* System (Blue/Yellow)
	* Network (Blue/Yellow)
*Buttons: 1x soft reset
*Power: 12V DC jack

Installation instructions:

Obtaining SSH access is mandatory
https://openwrt.org/inbox/toh/xiaomi/xiaomi_redmi_ax6_ax3000#ssh_access

Installation is done by the ubiformat method, through SSH:
1. Open an SSH shell to the router
2. Copy the file openwrt-ipq807x-generic-redmi_ax6-initramfs-factory.ubi to the /tmp directory
3. Check which rootfs partition is your router booted in (0 = rootfs | 1 = rootfs_1):
nvram get flag_boot_rootfs

4. Find the rootfs and rootfs_1 mtd indexes respectively:
cat /proc/mtd
Please confirm if mtd12 and mtd13 are the correct indexes from above!

5. Use the command ubiformat to flash the opposite mtd with UBI image:

If nvram get flag_boot_rootfs returned 0:
ubiformat /dev/mtd13 -y -f /tmp/openwrt-ipq807x-generic-redmi_ax6-initramfs-factory.ubi && nvram set flag_boot_rootfs=1 && nvram set flag_last_success=1 && nvram commit

otherwise:
ubiformat /dev/mtd12 -y -f /tmp/openwrt-ipq807x-generic-redmi_ax6-initramfs-factory.ubi && nvram set flag_boot_rootfs=0 && nvram set flag_last_success=0 && nvram commit

6. Reboot the device by:
reboot

Previous commands flashed an ubinized OpenWrt initramfs that will serve as the intermediate step
since OpenWrt uses unified rootfs in order to fully utilize NAND and provide enough space for packages.
Continue in order to pernamently flash OpenWrt:

7. SSH into OpenWrt from one of the LAN ports
8. Copy the file openwrt-ipq807x-generic-redmi_ax6-squashfs-sysupgrade.bin to the /tmp directory
9. Sysupgrade the device:
sysupgrade -n /tmp/openwrt-ipq807x-generic-redmi_ax6-squashfs-sysupgrade.bin

Device will reboot with OpenWrt, and then sysupgrade can be used to upgrade the device when desired.

Signed-off-by: Zhijun You <hujy652@gmail.com>