mediatek: add ABT ASR3000 support

mediatek: add ABT ASR3000 support

Hardware specification:
  SoC: MediaTek MT7981B 2x A53
  Flash: 128 MB SPI-NAND
  RAM: 256MB
  Ethernet: 4x 10/100/1000 Mbps
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset, Mesh
  Power: DC 12V 1A

Gain telnet access:
1. Login into web interface, and download the configuration.
2. Decode and uncompress the configuration:
     * Enter fakeroot if you are not login as root.
   base64 -d e-xxxxxxxxxxxx-cfg.tar.gz | tar -zx
3. Edit 'etc/passwd', remove root password: 'root::1:0:99999:7:::'.
4. Edit 'etc/rc.local', insert telnetd command before 'exit 0':
   ( sleep 3s; /usr/sbin/telnetd; ) &
5. Repack the configuration:
   tar -zc etc/ | base64 > e-xxxxxxxxxxxx-cfg.tar.gz
6. Upload new configuration via web interface, now you can connect to
   ASR3000 via telnet.

Flash instructions:
1. Connect to ASR3000, backup everything, especially 'Factory' part.
2. Write new BL2:
   mtd write openwrt-mediatek-filogic-abt_asr3000-preloader.bin BL2
3. Write new FIP:
   mtd write openwrt-mediatek-filogic-abt_asr3000-bl31-uboot.fip FIP
4. Set static IP on your PC:
   IP 192.168.1.254/24, GW 192.168.1.1
5. Serve OpenWrt initramfs image using TFTP server.
6. Cut off the power and re-engage, wait for TFTP recovery to complete.
7. After OpenWrt has booted, perform sysupgrade.

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Link: https://github.com/openwrt/openwrt/pull/15887
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>