target/arm: Make MPU_MAIR0, MPU_MAIR1 registers banked for v8M

Make the MPU registers MPU_MAIR0 and MPU_MAIR1 banked if v8M security
extensions are enabled.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1503414539-28762-13-git-send-email-peter.maydell@linaro.org
