Allow platform parameter X1 to be passed to BL3-1

bl2_main() was overwriting any platform set X1 parameter for BL3-1
with the value zero.

This patch ensure that any platform set value is correctly passed
to BL3-1. The FVP port adds a check to verify this parameter is
being passed correctly.

Fixes ARM-software/tf-issues#173

Change-Id: Ifbcda73d3d41d2b04a4baf5614e9d2d21f1717c8
diff --git a/plat/fvp/bl2_fvp_setup.c b/plat/fvp/bl2_fvp_setup.c
index e18cf7d..72580f9 100644
--- a/plat/fvp/bl2_fvp_setup.c
+++ b/plat/fvp/bl2_fvp_setup.c
@@ -156,6 +156,9 @@
  ******************************************************************************/
 struct entry_point_info *bl2_plat_get_bl31_ep_info(void)
 {
+#if DEBUG
+	bl31_ep_info->args.arg1 = FVP_BL31_PLAT_PARAM_VAL;
+#endif
 	return bl31_ep_info;
 }