Introduce GIC feature
    
    Some platforms, like aarch64, don't have APIC but GIC. So there's
    no reason to have <apic/> feature turned on. However, we are
    still missing <gic/> feature. This commit introduces the feature
    to XML parser and formatter, adds documentation and updates RNG
    schema.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>