Support SDEI on ARM platforms using frameworks implemented in earlier
patches by defining and exporting SDEI events: this patch defines the
standard event 0, and a handful of shared and private dynamic events.

Change-Id: I9d3d92a92cff646b8cc55eabda78e140deaa24e1
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>