ath79: add support for Ubiquiti EdgeSwitch/ToughSwitch 5XP

ath79: add support for Ubiquiti EdgeSwitch/ToughSwitch 5XP

The Ubiquiti ToughSwitch 5XP is a 5-port PoE Gigabit switch with a single
Fast-Ethernet management port. It supports both 24V passive PoE out on all
five ports.

Flash:    8 MB
RAM:      64 MB
SoC:      AR7242
Switch:   ar8327
USB:      1x USB 2.0
Ethernet: 5x GbE, 1x FE

Installation of the firmware is possible either via serial + tftpboot or
the factory firmware update function via webinterface.

By default the single Fast-Ethernet port labeled "MGMT" is configured
as the WAN port. Thus access to the device is only possible via the
five switch ports.

Serial: 3v3 115200 8n1

The serial header is located in the lower left corner of the switches PCB:

```
|
|
|
| o
| o RX
| o TX
| o GND
|
|
++  +-++-+  ++  ++  +
+--+ ++ +--++--++--+
```

Signed-off-by: Tobias Schramm <tobleminer@gmail.com>
[remove ubnt,sw compatible - fix spelling - wrap commit message -
remove superfluous phy-mode property]
Signed-off-by: David Bauer <mail@david-bauer.net>