target-arm: Add support for AArch32 S2 negative t0sz

Add support for AArch32 S2 negative t0sz. In preparation for
using 40bit IPAs on AArch32.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1445864527-14520-5-git-send-email-edgar.iglesias@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
