plat/arm: SPM: Force BL31 to DRAM when SPM is used

BL31 is running out of space, and the use-case of SPM doesn't require it
to be in SRAM. To prevent BL31 from running out of space in the future,
move BL31 to DRAM if SPM is enabled.

Secure Partition Manager design document updated to reflect the changes.

Increased the size of the stack of BL31 for builds with SPM.

The translation tables used by SPM in Arm platforms have been moved back
to the 'xlat_tables' region instead of 'arm_el3_tzc_dram'. Everything is
in DRAM now, so it doesn't make sense to treat them in a different way.

Change-Id: Ia6136c8e108b8da9edd90e9d72763dada5e5e5dc
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>