Correctly dimension the PSCI aff_map_node array

The array of affinity nodes is currently allocated for 32 entries
with the PSCI_NUM_AFFS value defined in psci.h. This is not enough
for large systems, and will substantially over allocate the array
for small systems.

This patch introduces an optional platform definition
PLATFORM_NUM_AFFS to platform_def.h. If defined this value is
used for PSCI_NUM_AFFS, otherwise a value of two times the number
of CPU cores is used.

The FVP port defines PLATFORM_NUM_AFFS to be 10 which saves
nearly 1.5KB of memory.

Fixes ARM-software/tf-issues#192

Change-Id: I68e30ac950de88cfbd02982ba882a18fb69c1445
diff --git a/docs/porting-guide.md b/docs/porting-guide.md
index d970190..f854af9 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -150,6 +150,11 @@
     Defines the maximum number of CPUs that can be implemented within a cluster
     on the platform.
 
+*   **#define : PLATFORM_NUM_AFFS**
+
+    Defines the total number of nodes in the affinity heirarchy at all affinity
+    levels used by the platform.
+
 *   **#define : PRIMARY_CPU**
 
     Defines the `MPIDR` of the primary CPU on the platform. This value is used