target/arm: Fetch GICv3 state directly from CPUARMState

target/arm: Fetch GICv3 state directly from CPUARMState

This eliminates the need for fetching it from el_change_hook_opaque, and
allows for supporting multiple el_change_hooks without having to hack
something together to find the registered opaque belonging to GICv3.

Signed-off-by: Aaron Lindsay <alindsay@codeaurora.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1523997485-1905-6-git-send-email-alindsay@codeaurora.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>