ath79: add support for TP-Link TL-MR6400

ath79: add support for TP-Link TL-MR6400

This device is an LTE router supported in ar71xx so far.
As per original commit, hardware specifications (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

Flashing instructions:
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.

Known issues:
- LTE module does not always come up during boot (showing USB enumeration errors). Similar behavior has been reported at least from one user for ar71xx, too. Turning USB off and on again will serve as a workaround.
- eth0 (LAN) always shows carrier as 1 even if no cable is plugged in (this works "correctly" on ar71xx)

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>
[several adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Filip Moc <lede@moc6.cz>