ACPI: APEI: Enable APEI multiple GHES source to share a single external IRQ

X-Gene platforms describe multiple GHES error sources with the same
hardware error notification type (external interrupt) and interrupt
number.

Change the GHES interrupt request to support sharing the same IRQ.

This change includs contributions from Tuan Phan <tphan@apm.com>.

Signed-off-by: Loc Ho <lho@apm.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>