ipq40xx: add support for EnGenius ENS620EXT

ipq40xx: add support for EnGenius ENS620EXT

Hardware
--------
CPU:   Qualcomm IPQ4018
RAM:   256M
FLASH: 32M SPI NOR W25Q256
ETH:   QCA8075
WiFi2: IPQ4018 2T2R 2SS b/g/n
WiFi5: IPQ4018 2T2R 2SS n/ac
LED:    - Power amber
        - LAN1(PoE) green
        - LAN2 green
        - Wi-Fi 2.4GHz green
        - Wi-Fi 5GHz green
BTN:    - WPS
UART:  115200n8 3.3V J1
       VCC(1) - GND(2) - TX(3) - RX(4)

Added basic support to get the device up and running for a sysupgrade
image only.
There is currently no way back to factory firmware, so this is a one-way
street to OpenWRT.
Install from factory condition is convoluted, and may brick your device:
1) Enable SSH and disable the CLI on the factory device from the web user
   interface (Management->Advanced)
2) Reboot the device
3) Override the default, limited SSH shell:
   a) Get into the ssh shell:
      ssh admin@192.168.1.1 /bin/sh --login
   b) Change the dropbear script to disable the limited shell. At the
      empty command prompt type:
        sed -i '/login_ssh/s/^/#/g’ dropbear
        /etc/init.d/dropbear restart
        exit
4) ssh in to a (now-) normal OpenWRT SSH session
5) Flash your built image
   a) scp openwrt-ipq40xx-engenius_ens620ext-squashfs-sysupgrade.bin
      admin@192.168.1.1:/tmp/
   b) ssh admin@192.168.1.1
   c) sysupgrade -n
      /tmp/openwrt-ipq40xx-engenius_ens620ext-squashfs-sysupgrade.bin
6) After flash completes (it may say "Upgrade failed" followed by
   "Upgrade completed") and device reboots, log in to newly flashed
   system. Note you will now need to ssh as root rather than admin.

Signed-off-by: Steve Glennon <s.glennon@cablelabs.com>
[whitespace fixes, reordered partitions, removed rng node from 4.14,
fixed 901-arm-boot-add-dts-files.patch]
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>