xlnx-zynqmp: Add a secure prop to en/disable ARM Security Extensions

Add a secure prop to en/disable ARM Security Extensions.
This is particularly useful for KVM runs.

Default to disabled to match the behavior of KVM.

This changes the default setup from having the ARM Security
Extensions to not longer having them.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1464173555-12800-2-git-send-email-edgar.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
