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/bl31_fvp_setup.c b/plat/fvp/bl31_fvp_setup.c
index 5169bd7..6554ec3 100644
--- a/plat/fvp/bl31_fvp_setup.c
+++ b/plat/fvp/bl31_fvp_setup.c
@@ -159,6 +159,7 @@
 	assert(from_bl2->h.version >= VERSION_1);
 
 	bl2_to_bl31_params = from_bl2;
+	assert(((unsigned long)plat_params_from_bl2) == FVP_BL31_PLAT_PARAM_VAL);
 #endif
 }