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>

--
Changes since v7:
 * Use __end_of_fixed_addresses as an arch-agnostic invalid fixmap entry
 * Move the locks definition into the function that uses them to make it
   clear these are NMI only.

Changes since v6:
 * Tinkering due to the absence of #ifdef
 * Added SDEI to the new ghes_is_synchronous() helper.

squash, embedded locks