iio:dac:ti-dac7612: Add driver for Texas Instruments DAC7612

It is a driver for Texas Instruments Dual, 12-Bit Serial Input
Digital-to-Analog Converter.

Datasheet of this chip:
http://www.ti.com/lit/ds/sbas106/sbas106.pdf



Signed-off-by: Ricardo Ribalda Delgado <ricardo@ribalda.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>