target-arm: Add S2 translation to 64bit S1 PTWs

Add support for applying S2 translation to 64bit S1
page-table walks.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1445864527-14520-12-git-send-email-edgar.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>