Introduce GIC feature

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>