iio:dac:ti-dac7311 Add driver for Texas Instrument DAC7311

It is a driver for Texas Instruments 8/10/12-bit 1-channel
compatible with DAC6311 and DAC5311 chips.

Datasheet of this chip:
http://www.ti.com/lit/ds/symlink/dac7311.pdf



Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>