pm: Do wfi on hotplug

For targets that do not have CONFIG_MSM_PM defined, the CPU never
executes any low power mode resulting in infinite loop. Do wfi on these
targets by default where no-pm driver is used.

Also, check for pending interrupts only if CONFIG_MSM_PM is enabled on
the target.

Change-Id: I9de9c579ea344c9b3cb3b797469fac6075a223b5
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>