arm64: acpi: Honour firmware_signature field of FACS, if it exists

arm64: acpi: Honour firmware_signature field of FACS, if it exists

If the firmware_signature changes then OSPM should not attempt to resume
from hibernate, but should instead perform a clean reboot. Set the global
swsusp_hardware_signature to allow the generic code to include the value
in the swsusp header on disk, and perform the appropriate check on resume.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>