ipq807x: add Spectrum SAX1V1K

ipq807x: add Spectrum SAX1V1K

Spectrum SAX1V1K is a AX WIFI router with 3 1G and 1 2.5G ports.
The router is provided to Spectrum customers.
It is OEM of Askey RT5010W
https://forum.openwrt.org/t/spectrum-sax1v1k-askey-rt5010w-openwrt-support/149923

It continues the original work by @MeisterLone to get this device supported.

Specifications:
```
  •  CPU: Qualcomm IPQ8072A Quad core Cortex-A53 2.2GHz
  •  RAM: 2048MB of DDR3
  •  Storage: 1024MB eMMC
  •  Ethernet: 3x 1G RJ45 ports (QCA8075) + 1 2.5G Port (QCA8081)
  •  WLAN:
     • 2.4GHz: Qualcomm QCN5024 4x4 802.11b/g/n/ax 1174 Mbps PHY rate
     • 5GHz: Qualcomm QCN5054 4x4 802.11a/b/g/n/ac/ax 2402 PHY rate

  •  LED: 1 gpio-controlled dual color led (blue/red)
  •  Buttons: 1x reset
  •  Power: 12V DC jack
```

Notes:
```
  •  This commit adds only single partition support, that means
     sysupgrade is upgrading the current rootfs partition.
  •  Installation can be done by serial connection only.
  •  A poulated serial header is onboard
     https://forum.openwrt.org/t/spectrum-sax1v1k-askey-rt5010w-openwrt-support/149923/6
  •  RX/TX is working, u-boot bootwait is active, secure boot is enabled.
```

Installation Instructions:

	**Most part of the installation is performed from an initramfs image.**

	Boot initramfs : Using serial connection
	1. Boot up the device and wait till it displays "VERIFY_IB: Success. verify IB ok"
	2. Once that message appears,
	    login with username 'root'
	    password serial number of your router in uppercase.
	3. Use vi to paste the 'open.sh' script from @MeisterLone github on your device
	    https://github.com/MeisterLone/Askey-RT5010W-D187-REV6/blob/master/Patch/open.sh
	4. chmod 755 open.sh
	5. ./open.sh
	6. Set your ip to 192.168.0.1
	7. Run a TFTP server and host the initramfs image on the TFTP server and name it "recovery.img"
	8. Reboot device. On boot it will try TFTP.

	Install OpenWrt from initramfs image:
	1. Use SCP (or other way) to transfer OpenWrt factory image
	2. Connect to device using SSH (on a LAN port)
	3. Flash firmware: sysupgrade
	   # sysupgrade -n -v /tmp/openwrt_sysupgrade.bin
	4. Set U-boot env variable: bootcmd
	   # fw_setenv bootcmd "run fix_uboot; run setup_and_boot"
	5. Reboot the device
	   # reboot
	6. Once device is booted, residue of previous firmware will prevent openwrt to work properly.
	    Factory Reset is MUST required
	    # Once serial console is displaying to login, hold reset button for 10 sec
	7. Now everything should be operational.

        Note: this PR adds only single partition support, that means sysupgrade is
              upgrading the current rootfs partition

Signed-off-by: Connor Yoon <j_connor@taliaent.com>