msm: krait-regulator: fix race conditions in regulator enable/disable

The driver suffers from the following race conditions:
* when a core is being onlined and is in its PREPARE_UP stage regulator
enable for that core is called. This call may cause a switch between
ldo-bhs but since the core if not online yet and ldo-bhs calls are
strictly executed on the target cpu - this leads to smp function calls
to fail.
* before the core in onlined and after the PREPARE_UP stage there could
be another frequency change that causes a gang voltage change that in turn
causes this core to switch states. This too leads to failed smp function
calls since they are targeted towards offline core
* when a cpu is offlined but a DEAD notification is not sent a frequency
change on another core may cause a switch on this cpu. This also leads
to failed smp function calls on offline core.

Fix the above by forcing the core to BHS when it is in its transitory
phase and while it is offlined. In other words a core will switch to
bhs/ldo based on the voltage requests only between CPU_ONLINE and
CPU_DOWN_PREPARE notifications. A core can run on BHS if it
can run on LDO since LDO just acts as a voltage step down, the core
can run when fed higher voltage from BHS.

To do this add hooks to get cpu hotplug notifications in the driver and
appropriately force bhs or switch to ldo or bhs based on gang voltage.

CRs-Fixed: 529593
Change-Id: I7210f62072853ed6f807df2d78bc2144f67eaf31
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>