Add an LED driver using the DAC124S085 DAC from NatSemi

[randy.dunlap@oracle.com: use header files for interfaces]
Signed-off-by: NGuennadi Liakhovetski <lg@denx.de>
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NRichard Purdie <rpurdie@linux.intel.com>