iio: dac: Add support for the AD5592R/AD5593R ADCs/DACs

This patch adds support for the AD5592R (spi) and AD5593R (i2c)
ADC/DAC/GPIO devices.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>