iio:dac:ad5686: Add support for AD5685R

This driver acctually supports the AD5685R device and NOT the AD5685, which
does not exist.  Maintain old id entry in case we have devicetrees
out there which are using the wrong name.

More information can be found in the datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/AD5686_5684.pdf

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>