ath79: add support for Ubiquiti ToughSwitch/EdgeSwitch 8XP

ath79: add support for Ubiquiti ToughSwitch/EdgeSwitch 8XP

Flash:    8 MB
RAM:      64 MB
SoC:      AR7242
Switch:   bcm53128
USB:      1x USB 2.0
Ethernet: 8x GbE, 1x FE

The Ubiquiti ToughSwitch 8XP is a 8-port PoE Gigabit switch with a single
Fast-Ethernet management port. It supports both 24V passive PoE and 48V
802.11af/at PoE out on all eight ports.

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 eight switch
ports.

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

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>
[fix whitespace issue]
Signed-off-by: David Bauer <mail@david-bauer.net>