staging:iio:dac: Add AD5360 driver

This patch adds support for the Analog Devices AD5360, AD5361, AD5362, AD5363,
AD5370, AD5371, AD5372, AD5373 multi-channel digital-to-analog converters.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>