Use xen dtb if xen image is present

Use xen dtb if xen image is present for foundation
and fvp pltaform.

Change-Id: I8ee0c5f59ea731202ca97c2cdef315cc5e6665e6
Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>