mediatek: filogic: add Qihoo 360T7 support

mediatek: filogic: add Qihoo 360T7 support

Hardware specification:
  SoC: MediaTek MT7981B 2x A53
  Flash: ESMT F50L1G41LB 128MB
  RAM: MT5CC128M16JR-EK 256MB
  Ethernet: 4x 10/100/1000 Mbps
  Switch: MediaTek MT7531AE
  WiFi: MediaTek MT7976C
  Button: Reset, WPS
  Power: DC 12V 1A

Flash instructions:
  1. Attach UART, boot the stock firmware until
     the message about failsafe mode appears.
  2. Enter failsafe mode by pressing "f" and "Enter"
  3. Type "mount_root", then run
     "fw_setenv bootmenu_delay 3"
  4. Back up all mtd partitions before flashing.
  5. Reboot, U-Boot now presents a menu.
  6. 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)
  7. Select "Upgrade ATF BL2", then use this file:
     openwrt-mediatek-filogic-qihoo_360t7-preloader.bin
  8. Select "Upgrade ATF FIP", then use this file:
     openwrt-mediatek-filogic-qihoo_360t7-bl31-uboot.fip
  9. Download the initramfs image, and type "reset",
     waiting for tftp recovery to complete.
  a. After openwrt boots up, perform sysupgrade.

Note:
  1. Since NMBM is disabled, we must back up all partitions.
  2. Flash instructions is based on commit 28df7f7.

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>