SPM: Register Secure Partition priority level with ehf module

Register a priority level, PLAT_SP_PRI, for secure partition with EL3
exception handling framework(ehf) module.

The secure partition manager(SPM) would raise the core's priority to
PLAT_SP_PRI before entering the secure partition, to protect the core
from getting interrupted while in secure partition.

Change-Id: I686897f052a4371e0efa9b929c07d3ad77249e95
Signed-off-by: Sughosh Ganu <sughosh.ganu@arm.com>