ACPI: tables: Add CEDT signature to the list of known tables

Add ACPI_SIG_CEDT to table_sigs[] in  "drivers/acpi/tables.c".

Signed-off-by: Robert Kiraly <me@boldcoder.com>
[ rjw: Rebase, new subject and changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>