plat/arm: Always define TSP memory region

Even though this is not used unless SPD=tspd, only defining it when
SPD_tspd is defined doesn't have any advantage and it makes it harder to
read the code.

Change-Id: I3d93135e05f39be071d16f8a47394a9a3ff54bc8
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>