target/arm: Make MPU_CTRL register banked for v8M

Make the MPU_CTRL register 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: 1503414539-28762-16-git-send-email-peter.maydell@linaro.org