ramips: mt76x8: add support for Yuncore M300

ramips: mt76x8: add support for Yuncore M300

Hardware:
 - SoC: MediaTek MT7628DAN (MIPS 580MHz)
 - Flash: 8 MiB Spansion S25FL064K
 - RAM: 64 MiB (built-into SoC)
 - WLAN: 2.4 GHz (MT7628)
 - Ethernet: 1x 10/100 Mbps WAN, 1x 10/100 LAN (MT7628)
 - Buttons: 1 Reset button
 - LEDs: 1x Red, 1x Green
 - Serial console: unpopulated header, 57600 8n1 (RX only)
 - Power: 12 VDC, 1 A

There are unpopulated areas on the board for 5 GHz WiFi via PCIe as well
as (most likely) Quectel EG25-G 4G module. As both are not populated on
my board support for both is missing for now.

Installation:
The installation can be done via the recovery HTTP server which is built
into the bootloader. Hold down the reset button while connecting the
device to power and keep holding a bit more than 3 seconds. Connect to
http://192.168.188.253/ and upload sysupgrade.bin file.

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