drivers: regulator: fix/improve usage of devicetree properties

Most of devicetree properties for regulator, such as:

- regulator-min/max-microvolt
- regulator-min/max-microamp
- regulator-allowed-modes
- etc.

Are meant to specify limits on what consumers may set. They are **NOT**
meant to describe the hardware capabilities. For example, I could have a
BUCK converter that supports 0-5V output voltage, but my circuit may
only allow working on the 2.7-3.3V range.

This patch reworks the API so that the API class layer manages this
information. This is done by drivers collecting all such fields in a
common configuration structure that is later accessed by the class
layer. This simplifies drivers implementation. For example, if A
consumer calls regulator_set_voltage() with a voltage that is supported
but not allowed, driver code won't be called. Similarly, if a regulator
is configured to be `always-on`, enable/disable driver code will never
be called.

Drivers have been adjusted. PCA9420 mode settings have been removed from
devicetree in this commit as they are not actual modes but PMIC states.
This will be refactored in a follow-up commit.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>