kirkwood: Allow nand to be configured via. devicetree

Add default configuration for NAND, to be enabled in your board config. Ensure
clock gating is set appropriately when the NAND is enabled.

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