mediatek: add support for JDCloud RE-CP-03

mediatek: add support for JDCloud RE-CP-03

Hardware specification:
  SoC: MediaTek MT7986A 4x A53
  Flash: 128GB eMMC
  RAM: 1GB DDR4
  Ethernet: 4x 1GbE, 1x 2.5GbE (RTL8221B)
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset, Joylink
  Power: DC 12V 2A

Flash instructions:
1. Download and flash the vendor migration firmware via webUI:
   https://firmware.download.immortalwrt.eu.org/cnsztl/mediatek/filogic/openwrt-mediatek-mt7986-jdcloud_re-cp-03-vendor-migration.bin
   (Default address is 192.168.68.1, user root, no password)
2. After device has booted up, write new GPT table:
   dd if=openwrt-mediatek-filogic-jdcloud_re-cp-03-gpt.bin of=/dev/mmcblk0 bs=512 seek=0 count=34 conv=fsync
3. Erase and write new BL2:
   echo 0 > /sys/block/mmcblk0boot0/force_ro
   dd if=/dev/zero of=/dev/mmcblk0boot0 bs=512 count=8192 conv=fsync
   dd if=openwrt-mediatek-filogic-jdcloud_re-cp-03-preloader.bin of=/dev/mmcblk0boot0 bs=512 conv=fsync
4. Erase and write new FIP:
   dd if=/dev/zero of=/dev/mmcblk0 bs=512 seek=13312 count=8192 conv=fsync
   dd if=openwrt-mediatek-filogic-jdcloud_re-cp-03-bl31-uboot.fip of=/dev/mmcblk0 bs=512 seek=13312 conv=fsync
5. Set static IP on your PC:
   IP 192.168.1.254/24, GW 192.168.1.1
6. Serve OpenWrt initramfs image using TFTP server.
7. Cut off the power and re-engage, wait for TFTP recovery to complete.
8. After OpenWrt has booted, perform sysupgrade.
9. Additionally, if you want to have eMMC recovery boot feature:
     (Don't worry! You will always have TFTP recovery boot feature.)
   dd if=openwrt-mediatek-filogic-jdcloud_re-cp-03-initramfs-recovery.itb of=/dev/mmcblk0p4 bs=512 conv=fsync

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>