target-arm: Add ARMCPU secure property

Added a "has_el3" state property to the ARMCPU descriptor.  This property
indicates whether the ARMCPU has security extensions enabled (EL3) or not.
By default it is disabled at this time.

Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1418684992-8996-10-git-send-email-greg.bellows@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
