iio: adc: rn5t618: Add ADC driver for RN5T618/RC5T619

Both chips have an A/D converter capable of measuring
things like VBAT, VUSB and analog inputs.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>