ipq40xx: add support for ZyXEL WRE6606

ipq40xx: add support for ZyXEL WRE6606

Specifications:
SOC:	Qualcomm IPQ4018 (DAKOTA) ARM Quad-Core
RAM:	128 MB Nanya NT5CC64M16GP-DI
FLASH:	16 MiB Macronix MX25L12845EMI-12G
ETH:	Qualcomm QCA8072
WLAN1:  Qualcomm Atheros QCA4018 2.4GHz 802.11b/g/n 2x2
WLAN2:  Qualcomm Atheros QCA4018 5GHz 802.11n/ac W2 2x2
INPUT:  WPS, Mode-toggle-switch
LED:	Power, WLAN 2.4GHz, WLAN 5GHz, LAN, WPS
        (LAN not controllable by software)
        (WLAN each green / red)
SERIAL:	Header next to eth-phy.
        VCC, TX, GND, RX (Square hole is VCC)
        The Serial setting is 115200-8-N-1.

Tested and working:
 - Ethernet (Correct MAC-address)
 - 2.4 GHz WiFi (Correct MAC-address)
 - 5 GHz WiFi (Correct MAC-address)
 - Factory installation from tftp
 - OpenWRT sysupgrade
 - LEDs
 - WPS Button

Not Working:
 - Mode-toggle-switch

Install via TFTP:

Connect to the devices serial. Hit Enter-Key in bootloader to stop
autobooting. Command `tftpboot` will pull an initramfs image named
`C0A86302.img` from a tftp server at `192.168.99.08/24`.
After successfull transfer, boot the image with `bootm`.

To persistently write the firmware, flash an openwrt sysupgrade image
from inside the initramfs, for example transfer
via `scp <sysupgrade> root@192.168.1.1:/tmp` and flash on the device
with `sysupgrade -n /tmp/<sysupgrade>`.

append-cmdline patch taken from chunkeeys work on the NBG6617.

Signed-off-by: Magnus Frühling <skorpy@frankfurt.ccc.de>
Co-authored-by: David Bauer <mail@david-bauer.net>
Co-authored-by: Christian Lamparter <chunkeey@googlemail.com>