qualcommax: ipq807x: add Asus RT-AX89X BX revision

qualcommax: ipq807x: add Asus RT-AX89X BX revision

This commit adds support for Asus RT-AX89X BX revision.

WARNING: Only the BX revision boards (So B1, B2 etc) are supported because
AX revision boards use IPQ8074 v1 SoC which is unsupported.

Specifications:
---------------
* CPU: Qualcomm IPQ8074A Quad core Cortex-A53 @ 2.2GHz
* RAM: 1024MB
* Storage: 256MB SLC NAND (Macronix MX30UF2G18AC)
* Ethernet:
	* 5x 1G RJ45 ports via QCA8337 switch
	* 3x 1G RJ45 ports via internal switch (QCA8075 PHY)
	* 1x 10G RJ45 via internal switch (AQR113C PHY)
	* 1x 10G SFP+ slot via internal switch
* WLAN:
	* 2.4GHz 4x4
	* 5GHz 8x8
	* 8 external antennas
* USB: 2x USB 3.0 Type-A
* Buttons:
	* Power switch
	* WPS
	* Reset
	* Wireless ON/OFF
	* LED ON/OFF
LED-s:
	* Power
	* Wi-Fi
	* WAN
	* 10G
	* SFP+
Power:
	* 19.5V via DC jack

Installation instructions:
--------------------------
1. Flash temporary OpenWrt initramfs:
* Flash openwrt-qualcommax-ipq807x-asus_rt-ax89x-initramfs-factory.trx
via the stock firmware.
Administration -> Firmware Upgrade -> Manual Firmware update (Upload)

After flashing the device will reboot with OpenWrt initramfs and it can
be accesed via any of the LAN ports via SSH with the usual OpenWrt
default credentials.

2. Sysupgrade from OpenWrt initramfs:
* Copy openwrt-qualcommax-ipq807x-asus_rt-ax89x-squashfs-sysupgrade.bin to
/tmp/openwrt-qualcommax-ipq807x-asus_rt-ax89x-squashfs-sysupgrade.bin of
the running initramfs image.
* Simply sysupgrade -n /tmp/openwrt-qualcommax-ipq807x-asus_rt-ax89x-squashfs-sysupgrade.bin

After flashing the device will reboot with OpenWrt initramfs and it can
be accesed via any of the LAN ports via SSH with the usual OpenWrt
default credentials.

Link: https://github.com/openwrt/openwrt/pull/15840
Signed-off-by: Robert Marko <robimarko@gmail.com>