mediatek: add factory image for ipTIME AX3000SM

mediatek: add factory image for ipTIME AX3000SM

Adds the capability to flash the factory image using the OEM recovery
software, ipTIME Firmware Wizard(11ac).

Installation
------------
1. Download the OEM recovery software from the manufacturer's website
2. Download the *squashfs-factory.bin file from the OpenWrt website
3. Press a reset button, and power up the router(keep pressing the reset button)
4. Wait more than 10 seconds until the CPU LED stop blinking
5. Connect the router(LAN port) to the PC
6. Run the OEM recovery software and follow the instructions
7. Select the *squashfs-factory.bin file during the router recovery process
8. Wait for the router to boot from *squashfs-factory.bin

Signed-off-by: Donghyun Ko <nyankosoftware@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19497
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>