ipq807x: add support for ZTE MF269

ipq807x: add support for ZTE MF269

Hardware specifications:
  SoC: Qualcomm IPQ8071A
  RAM: 512MB of DDR3
  Flash1: Eon EN25S64 8MB
  Flash2: MX30UF2G18AC 256MB
  Ethernet: 2x 2.5G RJ45 port
  Phone: 1x RJ11 port (SPI)
  USB: 1x Type-C 2.0 port
  WiFi1: QCN5024 2.4GHz
  WiFi2: QCN5054 5GHz
  Button: Reset, WPS

Flash instructions:
  1. Connect the router via serial port (115200 8N1 1.8V)
  2. Download the initramfs image, rename it to initramfs.bin,
     and host it with the tftp server.
  3. Interrupt U-Boot and run these commands:
     tftpboot initramfs.bin
     bootm
  4. After openwrt boots up, use scp or luci web
     to upload sysupgrade.bin to upgrade.

Signed-off-by: Chukun Pan <amadeus@jmu.edu.cn>
Reviewed-by: Robert Marko <robimarko@gmail.com>