ipq40xx: add support for Netgear EX6100v2/EX6150v2

ipq40xx: add support for Netgear EX6100v2/EX6150v2

Specifications:
SOC:	Qualcomm IPQ4018 (DAKOTA) ARM Quad-Core
RAM:	256 MB Winbond W632GU6KB12J
FLASH:	16 MiB Macronix MX25L12805D
ETH:	Qualcomm QCA8072
WLAN1:  Qualcomm Atheros QCA4018 2.4GHz 802.11b/g/n/ac 2x2
WLAN2:  Qualcomm Atheros QCA4018 5GHz 802.11n/ac
	1x1 (EX6100)
	2x2 (EX6150)
INPUT:  Power, WPS, reset button
	AP / Range-extender toggle
LED:	Power, Router, Extender (dual), WPS, Left-/Right-arrow
SERIAL:	Header next to QCA8072 chip.
	VCC, TX, RX, GND (Square hole is VCC)
	WARNING: The serial port needs a TTL/RS-232 v3.3 level converter!
        The Serial setting is 115200-8-N-1.

Tested and working:
 - Ethernet
 - 2.4 GHz WiFi (Correct MAC-address)
 - 5 GHz WiFi (Correct MAC-address)
 - Factory installation from WebIF
 - Factory installation from tftp
 - OpenWRT sysupgrade (Preserving and non-preserving)
 - LEDs
 - Buttons

Not Working:
 - AP/Extender toggle-switch

Untested:
 - Support on EX6100v2. They share the same GPL-Code and vendor-images.
   The 6100v2 seems to lack one 5GHz stream and differs in the 5GHz
   board-blob. I only own a EX6150v2, therefore i am only able to verify
   functionality on this device.

Install via Web-Interface:
Upload the factory image to the device to the Netgear Web-Interface.
The device might asks you to confirm the update a second time due to
detecting the OpenWRT firmware as older. The device will automatically
reboot after the image is written to flash.

Install via TFTP:
Connect to the devices serial. Hit Enter-Key in bootloader to stop
autobooting. Command "fw_recovery" will start a tftp server, waiting for
a DNI image to be pushed.
Assign your computer the IP-address 192.168.1.10/24. Push image with
tftp -4 -v -m binary 192.168.1.1 -c put <OPENWRT_FACTORY>
Device will erase factory-partition first, then writes the pushed image
to flash and reboots.

Parts of this commit are based on Thomas Hebb's work on the
openwrt-devel mailinglist.

See https://lists.openwrt.org/pipermail/openwrt-devel/2018-January/043418.html

Signed-off-by: David Bauer <mail@david-bauer.net>