ramips-mt76x8: add support for TP-Link RE305 v1

Specification:

SoC: MediaTek MT7628AN
RAM: 64MiB
Flash: 8MiB
Wifi:
  - 2.4GHz: MT7628AN
  - 5GHz: MT7612EN
LAN: 1x 10/100 Mbps

Flash instructions:
Flash factory image through stock firmware WEB UI.
Back to stock is possible by using TFTP and stripping down the Firmware
provided by TP-Link to a initramfs.

The flash space between 0x650000 and 0x7f0000
is blank in the stock firmware so I left it out as well.