acpi: add aml_while() term

Add encoding for ACPI DefWhile Opcode.

Reviewed-by: Shannon Zhao <zhaoshenglong@huawei.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>