target-arm: Use the right MMU index in arm_regime_using_lpae_format

arm_regime_using_lpae_format checks whether the LPAE extension is used
for stage 1 translation regimes. MMU indexes not exclusively of a stage 1
regime won't work with this method.

In case of ARMMMUIdx_S12NSE0 or ARMMMUIdx_S12NSE1, offset these values
by ARMMMUIdx_S1NSE0 to get the right index indicating a stage 1
translation regime.

Rename also the function to arm_s1_regime_using_lpae_format and update
the comments to reflect the change.

Signed-off-by: Alvise Rigo <a.rigo@virtualopensystems.com>
Message-id: 1452854262-19550-1-git-send-email-a.rigo@virtualopensystems.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
