ramips: add support for ipTIME A104ns

ramips: add support for ipTIME A104ns

ipTIME A104ns is a 2.4/5GHz band AC750 router, based on MediaTek MT7620A.

Specifications:
- SoC: MT7620A
- RAM: DDR2 64MB
- Flash: SPI NOR 8MB
- WiFi:
  - 2.4GHz: SoC internal
  - 5GHz: MT7610EN
- Ethernet: 5x 10/100Mbps
  - Switch: SoC internal
- USB: 1x 2.0
- UART:
  - J2: 3.3V, TX, RX, GND (3.3V is the square pad) / 57600 8N1

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.

In contrast to to-be-supported A1004ns, the A104ns has no usable
value in 0x1fc40 (uboot), so wan_mac needs to be calculated.
Also note that GPIOs for the LEDs really are inverted compared to
the A1004ns.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[moved state_default to device DTS, reordered properties in wmac,
added comment about wan_mac and LED GPIOs]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>