ARM: GIC: add GIC_INVALID to enum gic_version

ARM: GIC: add GIC_INVALID to enum gic_version

The enum gic_version at the moment just contains GIC_V2 and GIC_V3,
where GIC_V2 happens to map to 0. So without having initialised a
variable of that type, we will read back GIC_V2 (when allocated with zeroing
the memory).
To prevent ambiguities and to give an explicitly uninitialised state, add
a new first member: GIC_INVALID. Also make it obvious that this has a
"0" encoding.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Julien Grall <julien.grall@arm.com>