ramips: add support for Xiaomi MiWifi 3C

ramips: add support for Xiaomi MiWifi 3C

This commit adds support for Xiaomi MiWiFi 3C device.

Xiaomi MiWifi 3C has almost the same system architecture
as the Xiaomi Mi WiFi Nano, which is already officially
supported by OpenWrt.

The differences are:

 - Numbers of antennas (4 instead of 2). The antenna management
   is done via the µC. There is no configuration needed in the
   software code.
 - LAN port assignments are different. LAN1 and WAN are
   interchanged.

OpenWrt Wiki: https://openwrt.org/toh/xiaomi/mir3c

OpenWrt developers forum page:
https://forum.openwrt.org/t/support-for-xiaomi-mi-3c

Specifications:

 - CPU: MediaTek MT7628AN (575MHz)
 - 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
 - LED: 1x amber/blue/red. Programmable
 - Button: Reset

MAC addresses as verified by OEM firmware:

use address source
LAN *:92 factory 0x28
WAN *:92 factory 0x28
2g *:93 factory 0x4

OEM firmware uses VLAN's to create the network interface for WAN and LAN.

Bootloader info:
The stock bootloader uses a "Dual ROM Partition System".
OS1 is a deep copy of OS2.
The bootloader start OS2 by default.
To force start OS1 it is needed to set "flag_try_sys2_failed=1".

How to install:
1- Use OpenWRTInvasion to gain telnet, ssh and ftp access.
   https://github.com/acecilia/OpenWRTInvasion
   (IP: 192.168.31.1 - Username: root - Password: root)
2- Connect to router using telnet or ssh.
3- Backup all partitions. Use command  "dd if=/dev/mtd0 of=/tmp/mtd0".
   Copy /tmp/mtd0 to computer using ftp.
4- Copy openwrt-ramips-mt76x8-xiaomi_miwifi-3c-squashfs-sysupgrade.bin
   to /tmp in router using ftp.
5- Enable UART access and change start image for OS1.
```
nvram set uart_en=1
nvram set flag_last_success=1
nvram set boot_wait=on
nvram set flag_try_sys2_failed=1
nvram commit
```
6- Installing Openwrt on OS1 and free OS2.
```
mtd erase OS1
mtd erase OS2
mtd -r write /tmp/openwrt-ramips-mt76x8-xiaomi_miwifi-3c-squashfs-sysupgrade.bin OS1
```

Limitations: For the first install the image size needs to be less
than 7733248 bits.

Thanks for all community and especially for this device:
minax007, earth08, S.Farid

Signed-off-by: Eduardo Santos <edu.2000.kill@gmail.com>
[wrap lines, remove whitespace errors, add mediatek,mtd-eeprom to
 &wmac, convert to nvmem]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>