Introduce framework for CPU specific operations

This patch introduces a framework which will allow CPUs to perform
implementation defined actions after a CPU reset, during a CPU or cluster power
down, and when a crash occurs. CPU specific reset handlers have been implemented
in this patch. Other handlers will be implemented in subsequent patches.

Also moved cpu_helpers.S to the new directory lib/cpus/aarch64/.

Change-Id: I1ca1bade4d101d11a898fb30fea2669f9b37b956
diff --git a/bl31/bl31.ld.S b/bl31/bl31.ld.S
index 83ef7e7..add65b8 100644
--- a/bl31/bl31.ld.S
+++ b/bl31/bl31.ld.S
@@ -58,6 +58,15 @@
         KEEP(*(rt_svc_descs))
         __RT_SVC_DESCS_END__ = .;
 
+        /*
+         * Ensure 8-byte alignment for cpu_ops so that its fields are also
+         * aligned. Also ensure cpu_ops inclusion.
+         */
+        . = ALIGN(8);
+        __CPU_OPS_START__ = .;
+        KEEP(*(cpu_ops))
+        __CPU_OPS_END__ = .;
+
         *(.vectors)
         __RO_END_UNALIGNED__ = .;
         /*
@@ -69,6 +78,9 @@
         __RO_END__ = .;
     } >RAM
 
+    ASSERT(__CPU_OPS_END__ > __CPU_OPS_START__,
+           "cpu_ops not defined for this platform.")
+
     .data . : {
         __DATA_START__ = .;
         *(.data*)