Changed the fvp_config array in fvp_common.c into a struct and
moved into a new optional common platform header,
include/plat/common/plat_config.h. Removed the config definitions
in fvp_def.h and updated all references to the platform config.

This makes the interface to the platform config cleaner and uses
a little less RAM.

Fixes ARM-software/tf-issues#180

Change-Id: I58dd7b3c150f24f7ee230a26fd57c827853ba803