ramips: add support for WeVO AIR DUO

ramips: add support for WeVO AIR DUO

WeVO AIR DUO is a 1-bay NAS & 802.11ac (Wi-Fi 5) router, based on
MediaTek MT7620A.

Specifications:
* SoC: MT7620A
* RAM: 64 MiB
* Flash: SPI NOR 16 MiB
* USB & SATA bridge controller: JMicron JMS567
  * SATA 6Gb/s: 2.5" drive slot
  * USB 3.0: Micro-B
  * USB 2.0: connected to SoC
* Wi-Fi:
  * 2.4 GHz: SoC built-in
  * 5 GHz: MT7612EN
* Ethernet: 5x 1GbE
  * Switch: MT7530WU
* UART: 4-pin 1.27 mm pitch through-hole (57600 baud)
  * Pinout: (3V3)|(RXD) (TXD) (GND)

Notes:
* The drive is accessible through the external USB port only when the
  router is turned off.

Installation via web interface:
1.  Flash **initramfs** image through the stock web interface.
    The image filename should have ".upload" extension.
2.  Boot into OpenWrt and perform sysupgrade with sysupgrade image.

Revert to stock firmware:
1.  Perform sysupgrade with stock image.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>