ARM: PSCI: Allocate PSCI stack in secure stack section

Now that we have a secure stack section that guarantees usable memory,
allocate the PSCI stacks in that section.

Also add a diagram detailing how the stacks are placed in memory.

Reserved space for the target PC remains unchanged. This should be
moved to global variables within a secure data section in the future.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>