ar71xx: Add support for TP-Link MR6400

You can flash via tftp recovery (serve factory image as /mr6400_tp_recovery.bin
on 192.168.0.66/24, connect to any ethernet port and power on device while
holding the reset button). Flashing via OEM web interface does not work.

Hardware Specification (v1.0 EU):
 - SoC: QCA9531
 - Flash: Winbond W25Q64FV (8MiB)
 - RAM: EtronTech EM6AB160TSE-5G (64MiB)
 - Wireless: SoC platform only (2.4GHz b/g/n, 2x internal antenna)
 - Ethernet: 2NIC (3x100M + 1x100M)
 - WWAN: TP-LINK LTE MODULE (2x external detachable antenna)
 - Power: DC 12V 1A

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