ramips: add support for Netgear WAX214v2

ramips: add support for Netgear WAX214v2

Netgear WAX214v2 is an 802.11ax (Wi-Fi 6) wireless access point.

Specifications:
* SoC: MediaTek MT7621AT
* RAM: 512 MiB
* Flash: NAND 128 MiB (ESMT PSU1GA30DT)
* Wi-Fi: 2.4/5 GHz 4T4R (MediaTek MT7915E)
* Ethernet: 1x 10/100/1000 Mbps LAN
* Switch: MediaTek MT7530 (SoC built-in)
* LEDs/Keys
  * Power (green, blue, amber)
  * LAN (green, amber)
  * WLAN 2.4GHz (green, blue)
  * WLAN 5GHz (green, blue)
  * Reset button
* USB: None
* UART: Marked J1 on board, 115200 8N1
* Power
  * 12 VDC, 1.5 A
  * IEEE 802.3at (PoE+)

Load addresses (same as Netgear WAX202):
* stock
  * 0x80010000: FIT image
  * 0x81001000: kernel image -> entry
* OpenWrt
  * 0x80010000: FIT image
  * 0x82000000: uncompressed kernel+relocate image
  * 0x80001000: relocated kernel image -> entry

MAC addresses as verified by OEM firmware:

vendor   OpenWrt   address
eth0     lan       label
ra0      phy0      label + 2
rax0     phy1      label + 3

Installation:
* Flash the factory image by 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 to WAX214v2 GPL source:
https://www.downloads.netgear.com/files/GPL/WAX214v2-V1.0.1.5-gpl-src.tar.gz

* openwrt/target/linux/ramips/dts/mt7621-ax-nand-wax214v2.dts
  DTS file for this device.

Signed-off-by: Borys Zhukov <borys@zhukov.org>
Link: https://github.com/openwrt/openwrt/pull/14401
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>