arm: mvebu: Add GPIO LEDs to Mirabox board

Add the three external LED definitions to the device tree file on
the Mirabox.

The Mirabox user guide calls out one as a power LED, and the other
two are defined for WiFi, but as the current mwifiex drivers don't
have LED support, we make them status LEDs.

These have been tested working by writing to the appropriate
/sys/class/leds trigger.

Signed-off-by: Ryan Press <ryan@presslab.us>
Tested-by: Neil Greatorex <neil@fatboyfat.co.uk>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>