mediatek: add Nokia EA0326GMP support

mediatek: add Nokia EA0326GMP 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, WPS/Mesh
  Power: DC 12V 1A

Gain SSH access:
1. Login into web interface, and download the configuration.
2. Download the configration utilities:
   https://firmware.download.immortalwrt.eu.org/cnsztl/mediatek/filogic/openwrt-mediatek-mt7981-nokia-ea0326gmp-config-utils.tar.gz
     These binaries are extraced from the factory firmware, which are
     dynamically linked with aarch64 musl 1.1.24. To use them, you
     must run them under the same runtime environment, otherwise the
     binaries will not work properly!
3. Upload the configuration and utilities to a suitable environment.
4. Uncompress the utilities, move them to '/bin' and give them executable permisison:
   tar -zxf openwrt-mediatek-mt7981-nokia-ea0326gmp-config-utils.tar.gz
   mv mkconfig seama /bin
   chmod +x /bin/mkconfig
   chmod +x /bin/seama
5. Decrypt and uncompress the configuration:
     Enter fakeroot if you are not login as root.
   mkconfig -a de-enca -m EA0326GMP_3FE79221BAAA -i EA0326GMP_3FE79221BAAA-xxxxxxxx-backup.tar.gz -o backup.tar.gz
   tar -zxf backup.tar.gz
6. Edit 'etc/config/dropbear', set 'enable' to '1'.
7. Edit 'etc/passwd', remove root password: 'root::1:0:99999:7:::'.
8. Repack the configuration:
   tar -zcf backup.tar.gz etc/
   mkconfig -a enca -m EA0326GMP_3FE79221BAAA -i backup.tar.gz -o EA0326GMP_3FE79221BAAA-xxxxxxxx-backup.tar.gz
9. Upload new configuration via web interface, now you can SSH to EA0326GMP.

A minimum configuration which enabled SSH access is also provided
to simplify the process:
https://firmware.download.immortalwrt.eu.org/cnsztl/mediatek/filogic/openwrt-mediatek-mt7981-nokia-ea0326gmp-enable-ssh.tar.gz

Flash instructions:
1. SSH to EA0326GMP, backup everything, especially 'Factory' part.
2. Write new BL2:
   mtd write openwrt-mediatek-filogic-nokia_ea0326gmp-preloader.bin BL2
3. Write new FIP:
   mtd write openwrt-mediatek-filogic-nokia_ea0326gmp-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>