arm64: RME: Add MECID allocation

When MECID is supported, allocate a new private MEC for each Realm.
Userspace can opt to use the shared MECID (1) before Realm create.

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