SPM: Extract code out of the SMC handler

Simplify the code of the SMC handler by extracting the code of
SP_EVENT_COMPLETE and MM_COMMUNICATE.

Change-Id: I9250a3f5e4b807b35c9d044592c1074a45ab9a07
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>