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/aarch64/bl31_entrypoint.S b/bl31/aarch64/bl31_entrypoint.S
index a088c2e..a9238dc 100644
--- a/bl31/aarch64/bl31_entrypoint.S
+++ b/bl31/aarch64/bl31_entrypoint.S
@@ -68,7 +68,7 @@
 	 * Boot ROM(BL0) programming sequence
 	 * -----------------------------------------------------
 	 */
-	bl	cpu_reset_handler
+	bl	reset_handler
 #endif
 	/* ---------------------------------------------
 	 * Enable the instruction cache, stack pointer