ramips: add support for HiWiFi HC5861B

ramips: add support for HiWiFi HC5861B

HiWiFi "Gee Enjoy1200" HC5861B is a dual-band router based on MediaTek MT7628AN
https://www.hiwifi.com/enjoy-view

Specifications:
- MediaTek MT7628AN 580MHz
- 128 MB DDR2 RAM
- 16 MB SPI Flash
- 2.4G MT7628AN 802.11bgn 2T2R 300Mbps
- 5G MT7612EN 802.11ac 2T2R 867Mbps
- 5x 10/100 Mbps Ethernet

Flash instruction:
1. Get SSH access to the router
2. SSH to router with `ssh -p 1022 root@192.168.199.1`, The SSH password is the same as the webconfig one
3. Upload OpenWrt sysupgrade firmware into the router's `/tmp` folder with SCP
4. Run `mtd write /tmp/<filename> firmware`
5. reboot

Everything is working

Signed-off-by: Deng Qingfang <dengqf6@mail2.sysu.edu.cn>