iio: dac: dac5571: Support powerdown for multi-channel

The driver currently only allows channel 0 to be powered down but the
multi-channel variants of the hardware allow each channel to be powered
down separately and with separate power down modes.  Add support for
this.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Acked-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>