regulator: Add support for voltage aggregation

With this patch the regulator core aggregates voltage requests and
applies the minimum voltage range that satisifies all consumers of
a shared regulator supply. Only the voltages of consumers that have
enabled the supply are aggregated.

Updated voltage ranges are applied to the corresponding driver upon
the following events:

1. A consumer that has already enabled the regulator sets a voltage
2. A consumer enables the regulator
3. A consumer disables the regulator

Change-Id: Ib5012886cb77185a2038d45f08f3758faff68f60
Signed-off-by: David Collins <collinsd@codeaurora.org>