regulator: Add regulator_check_voltage function

[manu]

Summary

This function will call the regnode_check_voltage method for a given regulator and check
if the desired voltage in reachable by it.
Also adds a default method that check the std_param and which should be enough
for most regulators and add it as the method for axp* rk805 and fixed regulators.

Test Plan

Tested on Pine64-LTS and NanoPi-M1-Plus

[mmel]

LGTM