ramips: add support for XiaoYu XY-C5

Hardware:
SoC:      MT7621A
Flash:    32 MiB
RAM:      512 MiB
Ethernet: built-in switch
USB:      1x USB3.0
SATA:     ASM1060, 1 SATA port

Flash instruction (TFTP):
1. Set PC to fixed ip address 192.168.173.22
2. Download *-sysupgrade.bin image and rename it to firmware.img
3. Start a tftp server with the image file in its root directory
4. Turn off the router
5. Turn on router,press the reset button and wait ~15 seconds
6. Release the reset button and after a short time
   the firmware should be transferred from the tftp server
7. Wait ~3 minutes to complete recovery.

Signed-off-by: Qi Jiang <rushx@live.cn>
[squash commits, add label-mac-device, fix sign-off style]
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>