mediatek: add support for Netgear EX6250v2 series

mediatek: add support for Netgear EX6250v2 series

Netgear EX6250v2, EX6400v3, EX6410v2, EX6470 are wall-plug 802.11ac
(Wi-Fi 5) extenders. Like other MT7629 devices, Wi-Fi does not work
currently as there is no driver.

Related: https://github.com/openwrt/openwrt/pull/5084

For future reference, 2.4GHz MAC = LAN+1, 5GHz MAC = LAN+2.

Specifications:
* MT7629, 256 MiB RAM, 16 MiB SPI NOR
* MT7761N (2.4GHz) / MT7762N (5GHz) - no driver
* Ethernet: 1 port 10/100/1000
* UART: 115200 baud (labeled on board)

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.
* After installation, perform a factory reset. Wait for the device to
  boot, then hold the reset button for 10 seconds. This is needed
  because sysupgrade in the stock firmware will attempt to preserve its
  configuration using sysupgrade.tgz.
  See https://github.com/openwrt/openwrt/pull/4182

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

Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>
(cherry picked from commit 73de41898fcd06d837b013449c370c493bcdc595)