feat(fvp): spmd logical partition smc handler

This patch adds a basic el3 spmd logical partition to the fvp platform
via a platform specific smc handler. One of the use cases for el3
logical partitions is to have the ability to translate sip calls into
ff-a direct requests via the use of spmd logical partitions. The smc
handler creates a direct request based on the incoming smc parameters
and forwards the call as a direct request from the spmd logical
partition to the target secure partition.

Change-Id: If8ba9aab8203924bd00fc1dcdf9cd05a9a04a147