ad525x_dpot: add support for SPI parts

Split the bus logic out into separate files so that we can handle I2C and
SPI busses independently.  The new SPI bus logic brings in support for a
lot more parts:

	AD5160, AD5161, AD5162, AD5165, AD5200, AD5201, AD5203,
	AD5204, AD5206, AD5207, AD5231, AD5232, AD5233, AD5235,
	AD5260, AD5262, AD5263, AD5290, AD5291, AD5292, AD5293,
	AD7376, AD8400, AD8402, AD8403, ADN2850

[randy.dunlap@oracle.com: fix ad525X_dpot build]
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>