iio: adc: rn5t618: Add iio map

Add iio map to allow power driver to read out values as a consumer.
This approach does not block later addition of devicetree support
which would be helpful if there is an in-kernel consumer for AIN0/1.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>