hw/intc/arm_gic: Add Security Extensions property

hw/intc/arm_gic: Add Security Extensions property

Add a QOM property which allows the GIC Security Extensions to be
enabled. These are an optional part of the GICv1 and GICv2 architecture.
This commit just adds the property and some sanity checks that it
is only enabled on GIC revisions that support it.

Signed-off-by: Fabian Aggeler <aggelerf@ethz.ch>
Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1430502643-25909-3-git-send-email-peter.maydell@linaro.org
Message-id: 1429113742-8371-5-git-send-email-greg.bellows@linaro.org
[PMM: changed property name, added checks that it isn't set for
 older GIC revisions or if using the KVM VGIC; reworded commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>