spm-mm: Refactor secure_partition.h and its contents

Before adding any new SPM-related components we should first do
some cleanup around the existing SPM-MM implementation. The aim
is to make sure that any SPM-MM components have names that clearly
indicate that they are MM-related. Otherwise, when adding new SPM
code, it could quickly become confusing as it would be unclear to
which component the code belongs.

The secure_partition.h header is a clear example of this, as the
name is generic so it could easily apply to any SPM-related code,
when it is in fact SPM-MM specific.

This patch renames the file and the two structures defined within
it, and then modifies any references in files that use the header.

Change-Id: I44bd95fab774c358178b3e81262a16da500fda26
Signed-off-by: Paul Beesley <paul.beesley@arm.com>
diff --git a/docs/components/secure-partition-manager-design.rst b/docs/components/secure-partition-manager-design.rst
index de0792d..5c75baf 100644
--- a/docs/components/secure-partition-manager-design.rst
+++ b/docs/components/secure-partition-manager-design.rst
@@ -160,7 +160,7 @@
     Partition.
 
   - ``plat_get_secure_partition_boot_info()`` returns a
-    ``secure_partition_boot_info_t`` struct that is populated by the platform
+    ``spm_mm_boot_info_t`` struct that is populated by the platform
     with information about the memory map of the Secure Partition.
 
 For an example of all the changes in context, you may refer to commit