ramips: add support for GL-inet GL-MT300N-V2

This patch adds supports for the GL-inet GL-MT300N-V2.

Specification:
- SoC: MediaTek MT7628AN
- Flash: 16 MiB (W25Q128FVSG)
- RAM: 128 MiB DDR
- Ethernet: 1 x WAN (100 Mbps) and 1 x LAN (100 Mbps)
- USB: 1 x USB 2.0 port
- Button: 1 x switch button, 1 x reset button
- LED: 3 x LEDS (system power led is not GPIO controller)
- UART: 1 x UART on PCB (JP1: 3.3V, RX, TX, GND)

Installation through Luci:
- The original firmware is LEDE, so both LuCI or sysupgrade can be used.
- Do not keep settings, for sysupgrade please use the -n option.

Installation through bootloader webserver:
- Plug power and hold reset button until red LED blink to bright.
- Install sysupgrade image using web interface on 192.168.1.1.

Signed-off-by: Kyson Lok <kysonlok@gmail.com>
[match maximum image size with firmware partition]
Signed-off-by: Mathias Kresin <dev@kresin.me>