regulator: da9063: disable unused voltage monitors

Merge series from Benjamin Bara <bbara93@gmail.com>:

Follow-up for my initial patch regarding the disabling of unused
voltage monitors. We use the PWR_OK functionality, which asserts GP_FB2
if every monitored voltage is in range. This patch should provide the
possibility to deactivate a voltage monitor from the DT if the regulator
might be disabled during run time. For this purpose, the regulator
notification support is used:
https://lore.kernel.org/all/cover.1622628333.git.matti.vaittinen@fi.rohmeurope.com/