iio: adc: rockchip_saradc: add voltage notifier so get referenced voltage once at probe

Add voltage notifier, no need to query regulator voltage for
every saradc read, just get regulator voltage once at probe.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Simon Xue <xxm@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20210810011007.54066-1-xxm@rock-chips.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>