ARM platforms: Map TSP only when TSPD is included

This patch ensures that the ARM_MAP_TSP_SEC_MEM memory region is mapped
in BL2 only if the TSPD has been included in the build. This saves one
entry in the plat_arm_mmap[] array and avoids to map extra memory when
it's not needed.

Change-Id: I6ae60822ff8f0de198145925b0b0d45355179a94
Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>