leds: Add new driver for the LEDs on the Freecom FSG-3

The LEDs on the Freecom FSG-3 are connected to an external
memory-mapped latch on the ixp4xx expansion bus, and therefore cannot
be supported by any of the existing LEDs drivers.

Signed-off-by: Rod Whitby <rod@whitby.id.au>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>