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.
FixesARM-software/tf-issues#173
Change-Id: Ifbcda73d3d41d2b04a4baf5614e9d2d21f1717c8