ramips-mt76x8: add Xiaomi Mi Router 4C

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.

docs/user/supported_devices.rst

@@ -233,6 +233,7 @@ ramips-mt76x8
 * Xiaomi
 
   - Xiaomi Mi Router 4A (100M Edition)
+  - Xiaomi Mi Router 4C
 
 ramips-rt305x [#deprecated]_  [#device-class-tiny]_
 ---------------------------------------------------

targets/ramips-mt76x8

@@ -81,3 +81,7 @@ device('vocore2', 'vocore_vocore2', {
 device('xiaomi-mi-router-4a-100m-edition', 'xiaomi_mi-router-4a-100m', {
 	factory = false,
 })
+
+device('xiaomi-mi-router-4c', 'xiaomi_mi-router-4c', {
+	factory = false,
+})