filogic: add support for GL.iNet GL-MT6000

filogic: add support for GL.iNet GL-MT6000

Hardware specification:
* SoC: MediaTek MT7986A 4x A53
* Flash: 8GB EMMC
* RAM: 1GB DDR4
* Ethernet:
  * 2x2.5G RJ45 port (RTL8221B)
  * 4x1G RJ45 ports (MT7531AE)
* WLAN:
  * 2.4GHz: MT7976GN 4T4R
  * 5GHz: MT7976AN 4T4R
* Button: Reset
* LED: 1 x dual color LED
* USB: 1 x USB 3.0
* Power: DC 12V 4A
* UART: 3V3 115200 8N1 (Pinout: GND TX RX VCC)
* JTAG: 9 PIN

If you want to use u-boot from OpenWrt, you can upgrade it safely.
* bl2: openwrt-mediatek-filogic-glinet_gl-mt6000-preloader.bin
* fip: openwrt-mediatek-filogic-glinet_gl-mt6000-bl31-uboot.fip

`openwrt-mediatek-filogic-glinet_gl-mt6000-squashfs-factory.bin` is used in OpenWrt's u-boot.

Signed-off-by: Jianhui Zhao <zhaojh329@gmail.com>