Remove BL32_BASE when building without SPD for FVP

Remove BL32_BASE when building without SPD for FVP

Previously, when building TF without SPD support, BL2 tried to load a
BL32 image from the FIP and fails to find one, which resulted on
warning messages on the console. Even if there is a BL32 image in the
FIP it shouldn't be loaded because there is no way to transfer
control to the Secure Payload without SPD support.

The Makefile has been modified to pass a define of the form
SPD_${SPD} to the source code the same way it's done for PLAT. The
define SPD_none is then used to undefine BL32_BASE when BL32 is not
used to prevent BL2 from trying to load a BL32 image and failing,
thus removing the warning messages mentioned above.

Fixes ARM-software/tf-issues#287

Change-Id: Ifeb6f1c26935efb76afd353fea88e87ba09e9658