ramips: add support for ipTIME A3004T

ramips: add support for ipTIME A3004T

ipTIME A3004T is a 2.4/5GHz band router, based on Mediatek MT7621.

Specifications:
 - SoC: MT7621 (880MHz)
 - RAM: DDR3 256M
 - Flash: NAND 128MB  (Macronix NAND 128MiB 3,3V 8-bit)
 - WiFi:
   - 2.4GHz: MT7615E
   - 5GHz : MT7615E
 - Ethernet:
   - 4x LAN
   - 1x WAN
 - USB: 1 * USB3.0 port
 - UART:
   - 3.3V, TX, RX, GND / 57600 8N1

Installation via web interface:
 1. Flash initramfs image using OEM's Recovery mode
 2. Boot into OpenWrt and perform sysupgrade with sysupgrade image.

Revert to stock firmware:
 - Flash stock firmware via OEM's Recovery mode

How to use OEM's Recovery mode:
 1. Power up with holding down the reset key until CPU LED stop blinking.
 2. Set fixed ip with `192.168.0.2` with subnet mask `255.255.255.0`
 3. Flash image via tftp to `192.168.0.1`

Additional Notes:
 This router shares one MT7915E chip for both 2.4Ghz/5Ghz.
 radio0 will not working on 5Ghz as it's not connected to the antenna.

Signed-off-by: WonJung Kim <git@won-jung.kim>
(added led dt-bindings)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>