FVP: Include utils_def.h instead of utils.h

platform_def.h doesn't need all the definitions in utils.h,
the ones in utils_def.h are enough. This patch is related
to the changes introduced by commit 53d9c9c8.

Change-Id: I4b2ff237a2d7fe07a7230e0e49b44b3fc2ca8abe
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>