SPM: Move all SP-related info to SP context struct

Move all information related to a Secure Partition to the struct
secure_partition_context_t.

This requires an in-depth refactor because most of the previous code of
SPM relied on global information.

Change-Id: I0a23e93817dcc191ce1d7506b8bc671d376123c4
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>