This patch fixes a couple of issues for AArch32 builds on ARM reference
platforms :

1. The arm_def.h previously defined the same BL32_BASE value for AArch64 and
   AArch32 build. Since BL31 is not present in AArch32 mode, this meant that
   the BL31 memory is empty when built for AArch32. Hence this patch allocates
   BL32 to the memory region occupied by BL31 for AArch32 builds.

   As a side-effect of this change, the ARM_TSP_RAM_LOCATION macro cannot
   be used to control the load address of BL32 in AArch32 mode which was
   never the intention of the macro anyway.

2. A static assert is added to sp_min linker script to check that the progbits
   are within the bounds expected when overlaid with other images.

3. Fix specifying `SPD` when building Juno for AArch32 mode. Due to the quirks
   involved when building Juno for AArch32 mode, the build option SPD needed to
   specifed. This patch corrects this and also updates the documentation in the
   user-guide.

4. Exclude BL31 from the build and FIP when building Juno for AArch32 mode. As
   a result the previous assumption that BL31 must be always present is removed
   and the certificates for BL31 is only generated if `NEED_BL31` is defined.

Change-Id: I1c39bbc0abd2be8fbe9f2dea2e9cb4e3e3e436a8
Signed-off-by: Soby Mathew <soby.mathew@arm.com>