ramips: add support for Netgear EAX12 series

ramips: add support for Netgear EAX12 series

Netgear EAX12, EAX11v2, EAX15v2 are wall-plug 802.11ax (Wi-Fi 6)
extenders that share the SoC, WiFi chip, and image format with the
WAX202.

Specifications:
* MT7621, 256 MiB RAM, 128 MiB NAND
* MT7915: 2.4/5 GHz 2x2 802.11ax (DBDC)
* Ethernet: 1 port 10/100/1000
* UART: 115200 baud (labeled on board)

All LEDs and buttons appear to work without state_default.

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.

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

References in GPL source:
https://www.downloads.netgear.com/files/GPL/EAX12_EAX11v2_EAX15v2_GPL_V1.0.3.34_src.tar.gz

* target/linux/ramips/dts/mt7621-rfb-ax-nand.dts
  DTS file for this device.

Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>