ramips: add support for Netgear WAX202

ramips: add support for Netgear WAX202

Netgear WAX202 is an 802.11ax (Wi-Fi 6) router.

Specifications:
* SoC: MT7621A
* RAM: 512 MiB NT5CC256M16ER-EK
* Flash: NAND 128 MiB F59L1G81MB-25T
* Wi-Fi:
  * MT7915D: 2.4/5 GHz (DBDC)
* Ethernet: 4x 1GbE
  * Switch: SoC built-in
* USB: None
* UART: 115200 baud (labeled on board)

Load addresses (same as ipTIME AX2004M):
* stock
  * 0x80010000: FIT image
  * 0x81001000: kernel image -> entry
* OpenWrt
  * 0x80010000: FIT image
  * 0x82000000: uncompressed kernel+relocate image
  * 0x80001000: relocated kernel image -> entry

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.
* Note that the bootloader accepts both encrypted and unencrypted
  images, while the stock web interface only accepts encrypted ones.

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

References in WAX202 GPL source:
https://www.downloads.netgear.com/files/GPL/WAX202_V1.0.5.1_Source.rar

* openwrt/target/linux/ramips/dts/mt7621-ax-nand-wax202.dts
  DTS file for this device.

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