ad525x_dpot: add support for AD524x pots

New parts supported:
	AD5241, AD5242, AD5243, AD5245, AD5246, AD5247, AD5248

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>