ARM: kirkwood: Use devicetree to define DNS-32[05] fan

Remove more board-specific code by using devicetree to define the fan
attached to both boards.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>