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

Split platform.h into separate headers ...

Previously, platform.h contained many declarations and definitions
used for different purposes. This file has been split so that:

* Platform definitions used by common code that must be defined
  by the platform are now in platform_def.h. The exact include
  path is exported through $PLAT_INCLUDES in the platform makefile.

* Platform definitions specific to the FVP platform are now in
  /plat/fvp/fvp_def.h.

* Platform API declarations specific to the FVP platform are now
  in /plat/fvp/fvp_private.h.

* The remaining platform API declarations that must be ported by
  each platform are still in platform.h but this file has been
  moved to /include/plat/common since this can be shared by all
  platforms.

Change-Id: Ieb3bb22fbab3ee8027413c6b39a783534aee474a