Staging: iio: add driver for MAX517/518/519

IIO Driver for Maxim MAX517, MAX518 and MAX519 DAC

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>