It may be needed to enter the Secure Partition through other means than
an MM_COMMUNICATE SMC. This patch enables this behaviour by extracting
the necessary code from mm_communicate() and allowing other parts of the
code to use it.

Change-Id: I59f6638d22d9c9d0baff0984f39d056298a8dc8e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>