mediatek: filogic: add JCG Q30 PRO support

Hardware specification:
  SoC: MediaTek MT7981B 2x A53
  Flash: Winbond 128MB
  RAM: DDR3 256MB
  Ethernet: 4x 10/100/1000 Mbps
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset
  Power: DC 12V 1A

Flash instructions:
  1. Connect to your PC via the Gigabit port of the router,
     set a static ip on the ethernet interface of your PC.
     (ip 192.168.1.254, gateway 192.168.1.1)
  2. Attach UART, pause at u-boot menu.
  3. Select "Upgrade ATF BL2", then use preloader.bin
  4. Select "Upgrade ATF FIP", then use bl31-uboot.fip
  5. Download the initramfs image, and type "reset",
     waiting for tftp recovery to complete.
  6. After openwrt boots up, perform sysupgrade.

Note:
  1. Since NMBM is disabled, we must back up all partitions.
  2. Although we can upgrade new firmware in the stock firmware,
     we need the special fit image signature of MediaTek and
     dual boot (hack kernel) to make u-boot boot it. So just
     abandon these hacks and flash it via the serial port.

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
(cherry picked from commit 626344c9926dcf2db2e10681c19aab0328fee160)