ramips: add support for Xiaomi Mi Router 4C

ramips: add support for Xiaomi Mi Router 4C

This commit adds support for Xiaomi's Mi Router 4C device.

Specifications:

- CPU: MediaTek MT7628AN (580MHz)
- Flash: 16MB
- RAM: 64MB DDR2
- 2.4 GHz: IEEE 802.11b/g/n with Integrated LNA and PA
- Antennas: 4x external single band antennas
- WAN: 1x 10/100M
- LAN: 2x 10/100M
- LEDs: 2x yellow/blue. Programmable (labelled as power on case)
                      - Non-programmable (shows WAN activity)
- Button: Reset

How to install:

1- Use OpenWRTInvasion to gain telnet and ftp access.
2- Push openwrt firmware to /tmp/ using ftp.
3- Connect to router using telnet. (IP: 192.168.31.1 -
   Username: root - No password)
4- Use command "mtd -r write /tmp/firmware.bin OS1" to flash into
   the router..
5- It takes around 2 minutes. After that router will restart itself
   to OpenWrt.

Signed-off-by: Ataberk Özen <ataberkozen123@gmail.com>
[wrap commit message, bump PKG_RELEASE for uboot-envtools, remove
dts-v1 from DTS, fix LED labels]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>