arm64: Add MEC support

Most of the MEC support is handled by RMM. We can choose when
instantiating Realms whether we want one MEC context for each Realm or
use a shared MEC context between Realms.

There is a single shared MECID always available. Using it may be useful
if the platform has a very limited number of MECIDs (the max is 64k).

Signed-off-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>