UNTESTED firmware: arm_sdei: Add ACPI GHES registration helper

GHES event sources do not describe whether the event is shared or
private, which we must know to ensure normal and critical events
don't share locks or fixmap entries.

The helper asks firmware for this information so it can initialise
the struct ghes and register then enable the event.

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