mediatek: Routerich AX3000: add OpenWrt U-Boot layout

mediatek: Routerich AX3000: add OpenWrt U-Boot layout

This commit adds OpenWrt U-Boot layout support for Routerich AX3000. The
aims:
1. Get open-source U-Boot;
2. Get maximum available free space in OpenWrt.

Install
-------
1. Copy OpenWrt ubootmod-bl31-uboot.fip, ubootmod-preloader.bin, to the
   /tmp folder of the router using scp.

2. Make mtd partitions backups:
   http://192.168.1.1/cgi-bin/luci/admin/system/flash -> Save mtdblock
   contents

3. Install kmod-mtd-rw:
```
opkg update && opkg install kmod-mtd-rw
```

4. Write FIP and preloader:
```
insmod mtd-rw i_want_a_brick=1
mtd unlock BL2
mtd erase BL2
mtd write /tmp/ubootmod-preloader.bin BL2
mtd unlock FIP
mtd erase FIP
mtd write /tmp/ubootmod-bl31-uboot.fip FIP
```

5. Copy OpenWrt ubootmod-initramfs-recovery.itb to the tftp server root
   with IP 192.168.1.254.

6. Reboot router:
```
reboot
```
U-Boot will automatically download from the tftp server and boot OpenWrt
initramfs system.

7. Copy OpenWrt ubootmod-squashfs-sysupgrade.itb to the /tmp dir of the
   router using scp.

8. Run sysupgrade:
```
sysupgrade -n /tmp/squashfs-sysupgrade.itb
```

Recovery
--------
1. Place OpenWrt initramfs-recovery.itb image (with original name) on the
   tftp server (IP: 192.168.1.254).
2. Press "reset" button and power on the router. After ~10 sec release the
   button.
3. Use OpenWrt initramfs system for recovery.

BL2 and FIP recovery
--------------------
Use mtk_uartboot and UART connection if BL2 or FIP in UBI is destroyed:
Link: https://github.com/981213/mtk_uartboot
Return to stock:
----------------
1. Copy partition backups (BL2.bin and FIP.bin) to the /tmp dir of the
   router using scp.

2. Install kmod-mtd-rw:
```
opkg update && opkg install kmod-mtd-rw
```

3. Restore stock U-Boot and reboot:
```
insmod mtd-rw i_want_a_brick=1
mtd unlock BL2
mtd erase BL2
mtd write /tmp/BL2.bin BL2
mtd unlock FIP
mtd erase FIP
mtd write /tmp/FIP.bin FIP
reboot
```

4. Open U-Boot web recovery, upload stock firmware image and start
   upgrade.
Link: http://192.168.1.1
Signed-off-by: Mikhail Zhilkin <csharper2005@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16791
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>