hw/arm/virt: Enable TZ extensions on the GIC if we are using them

hw/arm/virt: Enable TZ extensions on the GIC if we are using them

If we're creating a board with support for TrustZone, then enable
it on the GIC model as well as on the CPUs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1441383782-24378-7-git-send-email-peter.maydell@linaro.org