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/include/lib/aarch64/arch.h b/include/lib/aarch64/arch.h
index 0427208..bb33acb 100644
--- a/include/lib/aarch64/arch.h
+++ b/include/lib/aarch64/arch.h
@@ -35,6 +35,8 @@
 /*******************************************************************************
  * MIDR bit definitions
  ******************************************************************************/
+#define MIDR_IMPL_MASK		0xff
+#define MIDR_IMPL_SHIFT		0x18
 #define MIDR_PN_MASK		0xfff
 #define MIDR_PN_SHIFT		0x4
 #define MIDR_PN_AEM		0xd0f