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The current secure partition design mandates that a) at a point, only
a single core can be executing in the secure partition, and b) a core
cannot be preempted by an interrupt while executing in secure
partition.

Ensure this by activating the SPM priority prior to entering the
parition. Deactivate the priority on return from the
partition.

Change-Id: Icb3473496d16b733564592eef06304a1028e4f5c
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>