regulator: core: Reduce busy-wait looping

Keep busy-wait looping to a minimum while waiting for a regulator to
ramp-up to the target voltage. This follows the guidelines set forth
in Documentation/timers/timers-howto.txt and assumes that regulators
are never enabled in atomic context.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>