ACPI / APEI: Add support for the SDEI GHES Notification type

If the GHES notification type is SDEI, register the provided event
using the SDEI-GHES helper.

SDEI may be one of two types of event, normal and critical. Critical
events can interrupt normal events, so these must have separate
fixmap slots and locks in case both event types are in use.

Signed-off-by: James Morse <james.morse@arm.com>