power: supply: rn5t618: Add voltage_now property

Read voltage_now via IIO and provide the property.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reported-by: kernel test robot <lkp@intel.com> # missing depends on IIO
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>