regulator: Add regulator_bulk_set_voltage

Add a convenience API to set the voltage on multiple consumers
stored in a struct regulator_bulk_data[] in one API call.

Change-Id: Iaeb5ba8c357a66f1401fb1e142bb03904e8e9c7c
Signed-off-by: Justin Paupore <jpaupore@codeaurora.org>