ramips: add support for TP-Link MR6400 v4

ramips: add support for TP-Link MR6400 v4

You can flash via tftp recovery:
 - serve tftp-recovery image as /tp_recovery.bin on 192.168.0.225/24
 - connect to any ethernet port
 - power on the device while holding the reset button
 - wait at least 8 seconds before releasing reset button

Flashing via OEM web interface does not work.

LTE module does not support DHCP so it must be configured via QMI.

Hardware Specification (v4.0 EU):
 - SoC: MT7628NN
 - Flash: Winbond W25Q64JVS (8MiB)
 - RAM: ESMT M14D5121632A (64MiB)
 - Wireless: SoC platform only (2.4GHz b/g/n, 2x internal antenna)
 - Ethernet: 1NIC (4x100M)
 - WWAN: TP-LINK LTE MODULE (2x external detachable antenna)
 - Power: DC 9V 0.85A

Signed-off-by: Filip Moc <lede@moc6.cz>