mediatek: add support for Netgear WAX206

mediatek: add support for Netgear WAX206

Specifications:
* SoC: MediaTek MT7622BV
* RAM: DDR3 512 MiB (Nanya NT5CC256M16ER-EK)
* Flash: SPI-NAND 256 MiB (Toshiba TC58CVG1S3HRAIJ)
* Wi-Fi 2.4/5 GHz 4T4R:
  * 2.4 GHz: MediaTek MT7622BV
  * 5 GHz: MediaTek MT7915AN/MT7975AN
* Ethernet: 4x 10/100/1000 Mbps LAN,
            1x 10/100/1000/2500 Mbps WAN (Realtek RTL8221B PHY)
* Switch: MediaTek MT7531AE
* LEDs/Keys: 8/1 (Power, Internet, LAN1, LAN2, LAN3, LAN4,
             Wifin and Wifia dual-colour LEDs + Reset pin)
* UART: Marked J19 on board VCC GND TX RX, beginning from "1". 3.3v,
        115200n8
* Power: 12 VDC, 2.5 A

Installation:
* Flash the factory image through the stock web interface, or TFTP to
  the bootloader. NMRP can be used to TFTP without opening the case.
* U-Boot allows booting an initramfs image via TFTP as follows:
  setenv ipaddr 192.168.1.1
  setenv serverip 192.168.1.100
  tftpboot openwrt-mediatek-mt7622-netgear_wax206-initramfs-recovery.itb
  bootm

Known Limitations:
* The 2.5G WAN port labeled 'wan' only works for speeds up to 1G at the
  moment. If connected to a multi-gig port the speed has to be manually
  set to 1G/full either for the switch port or in OpenWrt. For example
  add the following to /etc/rc.local to set it on boot:
  /usr/sbin/ethtool -s wan speed 1000 duplex full

Revert to stock firmware:
* Flash the stock firmware to the bootloader using TFTP/NMRP.

References to WAX206 GPL source:
https://www.downloads.netgear.com/files/GPL/WAX206_V1.0.4.0_Source.rar

* openwrt/target/linux/mediatek/dts/mt7622-netgear-wax206.dts
  DTS file for this device.
* openwrt/target/linux/mediatek/image/mt7622.mk
  Image creation code for this device

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
[fix WAN port (1G only), adjust partition layout, adjust image creation]
Signed-off-by: Thomas Kupper <thomas.kupper@gmail.com>