boards: add support for NEWIFI-Y1S

[tpishere]

Dear @hanwckf
I would appreciate if you would also support the NEWIFI-Y1S router in your repository.

The NEWIFI-Y1S has almost the same system architecture as the NEWIFI-MINI, which you are already supporting in your repository:

CPU:MediaTek MT7620A 
RAM MB:256
Ethernet 100M ports:3 
Ethernet Gbit ports:2 
WLAN Hardware:MediaTek MT7612E 
WLAN 2.4GHz:b/g/n 
WLAN 5.0GHz:a/n/ac 
USB ports:3x 2.0

The differences to NEWIFI-MINI are:

RAM size: 256 MB instead of 128 MB 
WAN/LAN4/LAN3/LAN2(Gbit)/LAN1(Gbit) port assignments 
USB ports:3x 2.0