Allow FP register context to be optional at build time

CTX_INCLUDE_FPREGS make variable allows us to include or exclude FP
registers from context structure, in case FP is not used by TSPD.

Fixes ARM-software/tf-issues#194

Change-Id: Iee41af382d691340c7ae21830ad1bbf95dad1f4b
diff --git a/bl31/aarch64/context.S b/bl31/aarch64/context.S
index 6667419..2698215 100644
--- a/bl31/aarch64/context.S
+++ b/bl31/aarch64/context.S
@@ -293,6 +293,7 @@
  * TODO: Revisit when VFP is used in secure world
  * -----------------------------------------------------
  */
+#if CTX_INCLUDE_FPREGS
 	.global fpregs_context_save
 func fpregs_context_save
 	stp	q0, q1, [x0, #CTX_FP_Q0]
@@ -368,3 +369,4 @@
 	 */
 
 	ret
+#endif /* CTX_INCLUDE_FPREGS */