regulator: unexport regulator_lock/unlock()

regulator_lock/unlock() was used only to guard
regulator_notifier_call_chain(). As no users remain, make the functions
internal.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lore.kernel.org/r/d3381aabd2632aff5e7b839d55868bec6e85c811.1600550732.git.mirq-linux@rere.qmqm.pl


Signed-off-by: Mark Brown <broonie@kernel.org>