ramips: add support for ipTIME A8004T

ramips: add support for ipTIME A8004T

ipTIME A8004T is a 2.4/5GHz band AC2600 router, based on Mediatek
MT7621A.

Specifications:
- SoC: MT7621A
- RAM: DDR3 256M
- Flash: SPI NOR 16MB
- WiFi:
  - 2.4GHz: MT7615E
  - 5GHz: MT7615E
- Ethernet: 5x 10/100/1000Mbps
  - Switch: SoC internal
- USB: 1 * USB3.0 port
- UART:
  - J4: 3.3V, TX, RX, GND (3.3V is the square pad) / 57600 8N1
- Other info:
  - J9: Unknown unpopulated header.

Installation via web interface:
1.  Flash **initramfs** image through the stock web interface.
2.  Boot into OpenWrt and perform sysupgrade with sysupgrade image.

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

Signed-off-by: Yong-hyu Ban <perillamint@quendi.moe>
[do not enable xhci node in DTS which is already enabled in DTSI]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>