ramips: add support for HiWiFi HC5761A

ramips: add support for HiWiFi HC5761A

HiWiFi HC5761A is an "MT7628AN variant" of HC5761

Specifications:
- MediaTek MT7628AN 580MHz
- 128 MB DDR2 RAM
- 16 MB SPI Flash
- 2.4G MT7628AN 802.11bgn 2T2R 300Mbps
- 5G MT7610EN 802.11ac 433Mbps
- 3x 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

Known bug:
- SD slot does not work (See PR 1500)

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