iio: adc: modify NPCM reset support

Modify NPCM ADC reset support from
direct register access to reset controller support.

please make sure to modify NPCM adc device tree
parameters as described at nuvoton,npcm-adc.txt
document for using this change.

Note for anyone noting that this is a breaking change, this is on
a BMC and effectively in a close ecosystem so it is fine to rely
on DT and kernel being updated together.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>