ramips: add Xiaomi Mi Router 3G support

ramips: add Xiaomi Mi Router 3G support

This commit adds support for Xiaomi Mi WiFi Router 3G.

Short specification:
 - MT7621AT + MT7603EN + 7612EN
 - 256MB DDR3 RAM
 - 128MB NAND flash
 - 1+2 x 1000M Ethernet
 - 1x USB 3.0 port
 - reset button
 - yellow, blue, red leds

Installation through telnet/ssh:
- copy lede-ramips-mt7621-mir3g-squashfs-kernel1.bin and
lede-ramips-mt7621-mir3g-squashfs-rootfs0.bin to usb disk or wget it
from LEDE download site to /tmp
- switch to /extdisks/sda1/ (if copied to USB drive) or to /tmp if
wgetted from LEDE download site
- run: mtd write lede-ramips-mt7621-mir3g-squashfs-kernel1.bin kernel1
- run: mtd write lede-ramips-mt7621-mir3g-squashfs-rootfs0.bin rootfs0
- run: mtd erase kernel0
- run: reboot

Originally stock firmware has following partitions:
 - ...
 - kernel0 (primary kernel image)
 - kernel1 (secondary kernel image, used by u-boot in failsafe routine)
 - rootfs0 (primary rootfs)
 - rootfs1 (secondary rootfs in case primary fails)
 - overlay (used as ubi overlay)

This commit squashes rootfs0, rootfs1 and overlay partitions into 1, so
it can be used by LEDE fully for package installation, resulting in 117,5MiB.

This device lacks hw watchdog, so adding softdog instead (stock does the same).

Signed-off-by: Pavel Kubelun <be.dissent@gmail.com>