ar71xx: add support for EnGenius ENS202EXT

ar71xx: add support for EnGenius ENS202EXT

EnGenius ENS202EXT is an outdoor wireless access point with
2-port 10/100 switch, detachable antennas and proprietery PoE.

The device is based on Qualcomm/Atheros AR9341 v1.

Specifications:
- 535/400/200 MHz (CPU/DDR/AHB)
- 64 MB of RAM
- 16 MB of FLASH
- UART (J1) header on PCB (unpopulated)
- 2x 10/100 Mbps Ethernet
- 2.4 GHz, up to 26dBm
- 2x external, detachable antennas
- 7x LED, 1x button

Flash instructions:

You have three options:

- Use the vendor firmware upgrade page on the web interface and give
it the factory.img. This is the easiest way to go about it.

- If you have serial access during u-boot, interrupt the normal boot
(any key before timeout) and run 'run failsafe_boot'; this will bring
you to a minimal openwrt luci image on ip 192.168.1.1 useful if you've
bricked the normal firmware.

- Use the vendor's management cli, which can be accessed via telnet
with the same credentials as the web login (default admin:admin), then
issue the following commands:

*** Hi admin, welcome to use cli(V-1.6.7) ***
---========= Commands Help =========---
      stat -- Status
       sys -- System
    wless2 -- 2.4G-Wireless
      mgmt -- Management
      tree -- Tree
      help -- Help
    reboot -- Reboot
ens202ext>mgmt
  Management
---========= Commands Help =========---
     admin -- Administration
     mvlan -- Management VLAN settings
      snmp -- SNMP settings
    backup -- Backup/Restore settings
    autorb -- Auto reboot settings
   fwgrade -- Firmware upgrade
      time -- Time settings
   wifisch -- Wifi schedule
       log -- Log
      diag -- Diagnostics
      disc -- Device Discovery
    logout -- Logout
      help -- Help
      exit -- Exit
ens202ext/mgmt>fwgrade
  Management --> Firmware upgrade
---========= Commands Help =========---
      fwup -- Firmware upgrade
      help -- Help
      exit -- Exit
ens202ext/mgmt/fwgrade>fwup http://web.server/lede-ar71xx-generic-ens202ext-squashfs-factory.bin

Signed-off-by: Marty Plummer <ntzrmtthihu777@gmail.com>