msm: krait-regulator: Adjust uA and uV aggregation for offlined rails

When a core is offlined, it's voltage and current contributions
should be removed from the aggregation that's performed when
setting the voltage and load requirements. This allows our ganged
regulator voltage and phase count to remain no higher than
necessary after we CPU hotplug a core.

In order to support this, introduce regulator_enable() and
regulator_disable() callbacks so that it's possible to control
whether a vreg is in the aggregation list. Also take care to
apply any voltage or current requirements for both disable and
enable. On disable, this allows for lowering the aggregated
voltage and current requirements; on enable, this allows any
changed requirements while the regulator was disabled to be
applied.

Also introduce an is_enabled() callback, since this is required
to support enable().

Change-Id: Ibec8f0182e1edd579f5780d7cc204152491e56fd
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>