iio: dac: Add Texas Instruments 8/10/12-bit 2/4-channel DAC driver

iio: dac: Add Texas Instruments 8/10/12-bit 2/4-channel DAC driver

The DACrrcS085 (rr = 08/10/12, c = 2/4) family of SPI DACs was
inherited by TI when they acquired National Semiconductor in 2011.
This driver was developed for and tested with the DAC082S085 built into
the Revolution Pi by KUNBUS, but should work with any of the other
chips as they share the same programming interface.

There is also a family of I2C DACs with just a single channel called
DACrr1C08x (rr = 08/10/12, x = 1/5).  Their programming interface is
very similar and it should be possible to extend the driver for these
chips with moderate effort.  Alternatively they could be integrated into
ad5446.c.  (The AD5301/AD5311/AD5321 use different power-down modes but
otherwise appear to be comparable.)

Furthermore there is a family of 8-channel DACs called DACrr8S085
(rr = 08/10/12) as well as two 16-bit DACs called DAC161Sxxx
(xxx = 055/997).  These are more complicated devices with support for
daisy-chaining and the ability to power down each channel separately.
They could either be handled by a separate driver or integrated into the
present driver with a larger effort.

Cc: Mathias Duckeck <m.duckeck@kunbus.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>