regulator: Add regulator_check_voltage function

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.

Reviewed by: mmel
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D22260