fix(el3-spmc): check descriptor size for overflow

Ensure that the provided descriptor size used when reserving space
for a memory descriptor does not overflow to prevent scope for
memory corruption. Reported by Matt Oh, Google Android Red Team.

Reported-by:  <mattoh@google.com>
Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: If06985c4de9a88ff82ce60d10e346da948ed383f