feat(sdei): add a function to return total number of events registered

This patch adds a public API to return the total number of registered
events. The purpose of this is primarily for DRTM to ensure that no
SDEI event can interfere with a dynamic launch.

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: I1d1cba2da7d5566cc340620ee1ce7d7844740b86
2 files changed