msm: hotplug: wait for hotplug completion event from secondary core

Primary core has to wait for hotplug completion event from secondary
core to avoid race between ongoing hotplug and already queued online
event. This also prevent CPU_DEAD notifiers from executing before the
SPM has completed the collapse of the CPU. CPU DEAD notifiers executing
early have been attributed to MDD hardware block for the CPU being
turned off prematurely, resulting in crashes.

The patch also adds support for probing with device tree for 8974
targets.

Change-Id: I8c155324adfee35825181bbf7b28a52b87ca2938
CRs-fixed: 438314,457541
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>