ramips: add support for Hi-Link HLK-7688A

ramips: add support for Hi-Link HLK-7688A

Specifications:

  - SoC: MediaTek MT7688AN
  - RAM: 128 MB
  - Flash: 32 MB
  - Ethernet: 5x 10/100 (1x WAN, 4x LAN)
  - Wireless: built in 2.4GHz (bgn)
  - USB: 1x USB 2.0 port
  - Buttons: 1x Reset
  - LEDs: 1x (WiFi)

Flash instructions:

  - Configure TFTP server with IP address 10.10.10.3
  - Name the firmware file as firmware.bin
  - Connect any Ethernet port to the TFTP server's LAN
  - Choose option 2 in U-Boot
  - Alternatively choose option 7 to upload firmware to the built-in
    web server

MAC addresses as verified by OEM firmware:

  use   address   source
  2g    *:XX      factory 0x4
  LAN   *:XX+1    factory 0x28
  WAN   *:XX+1    factory 0x2e

Notes:

This board is ostensibly a module containing the MediaTek MT7688AN SoC,
128 MB DDR2 SDRAM and 32 MB flash storage.  The SoC can be operated in
IoT Gateway Mode or IoT Device Mode.

From some vendors the U-Boot that comes installed operates on UART 2
which is inaccessible in gateway mode and operates unreliably in the
Linux kernel when using more than 64 MB of RAM.  For those, updating
U-Boot is recommended.

Signed-off-by: Ewan Parker <ewan@ewan.cc>
[add WLAN to 01_leds]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>