staging:iio:dac: Add AD5764 driver

This patch adds support for the Analog Devices AD5764, AD5764R, AD5744, AD5744R
quad channel analog-to-digital converter.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>