mediatek: add support for ipTIME A6004MX Add basic support for ipTIME A6004MX.

mediatek: add support for ipTIME A6004MX Add basic support for ipTIME A6004MX.

Hardware:
SoC: MediaTek MT7629 Cortex-A7 (ARMv7 1.25GHz, Dual-Core)
RAM: DDR3 128MB
Flash: Macronix MX35LF1GE4AB (SPI-NAND 128MB)
WiFi: MediaTek MT7761N (2.4GHz) / MediaTek MT7762N (5GHz) - no driver
Ethernet: SoC (WAN) / MediaTek MT7531 (LAN x4)
UART: [GND, RX, TX, 3.3V] (115200)

Installation:
- Flash recovery image with TFTP recovery

Revert to stock firmware:
- Flash stock firmware with TFTP recovery

TFTP Recovery method:
1. Unplug the router
2. Hold the reset button and plug in
3. Release when the power LED stops flashing and go off
4. Set your computer IP address manually to 192.168.0.x / 255.255.255.0
5. Flash image with TFTP client to 192.168.0.1

Signed-off-by: Yoonji Park <koreapyj@dcmys.kr>