ramips: add support for HILINK HLK-7628N

ramips: add support for HILINK HLK-7628N

Specification:

CPU:       MT7628 580 MHz. MIPS 24K
RAM:       128 MB
Flash:     32 MB
WIFI:      802.11n/g/b 20/40 MHz
Ethernet:  5 Port ethernet switch
UART:      2x

Flash instruction:
The U-boot is based on Ralink SDK so we can flash the firmware using UART:
1. Configure PC with a static IP address and setup an TFTP server.
2. Put the firmware into the tftp directory.
3. Connect the UART0 line as described on the PCB.
4. Power up the device and press 2, follow the instruction to
   set device and tftp server IP address and input the firmware
   file name. U-boot will then load the firmware and write it into
   the flash.
5. After firmware is started connect via ethernet at 192.168.1.1

Signed-off-by: Liu Yu <f78fk@live.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [removed dupped subject]