ramips: add support for YunCore G720

ramips: add support for YunCore G720

The YunCore G720 is a dual band 802.11ax router with 5 GbE ports.

Specs:
- SoC: MediaTek MT7621
- Ethernet: 5x GbE ports (built-in MT7530)
- Wireless 2.4GHz / 5GHz: MediaTek MT7915E
- RAM: 256MiB
- ROM: 16MiB (W25Q128)
- 1 Button (reset)
- 8 LEDs (1x system, 2x wifi, 5x switch ports)

Flash instructions:
The vendor firmware is based on OpenWrt, the sysupgrade image can be
flashed using the '-F' (force) option on the CLI.
Make sure not to keep settings when doing so.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>