regulator: Disable unused regulator

bootloaders such as u-boot might enable regulators, or simply regulators could
be enabled by default by the PMIC, even if we don't have a driver for
the device or subsystem.
Disable unused regulators just before going to userland.
A tunable hw.regulator.disable_unused is added to not disable them in case
this causes problems on some board but the default behavior is to disable
everything unused.
I prefer to break thinks now and fix them rather than never switch to the
case were we disable regulators.

Tested on : Pine64-LTS (an idle board goes from ~0.33A to ~0.27A)
Tested on : BananaPi M2
Differential Revision: https://reviews.freebsd.org/D14781