regulator: onsemi-ncp6335d: add list_voltage callback function

This callback function can help perform validation of the
regulator-min-microvolt and regulator-max-microvolt properties at runtime
automatically.

Change-Id: I63917e06c4fa881f67d938d0c53e7ad71ebda25f
Signed-off-by: Ke Liu <keliu@codeaurora.org>