hw/arm/exynos4210: Put external GIC into state struct

Switch the creation of the external GIC to the new-style "embedded in
state struct" approach, so we can easily refer to the object
elsewhere during realize.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220404154658.565020-9-peter.maydell@linaro.org