ACPI: tables: Add AGDI to the list of known table signatures

Add AGDI to the list of known ACPI table signatures to allow the
kernel to recognize it when upgrading tables via initrd.

Signed-off-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>