target/arm: Make MPU_RBAR, MPU_RLAR banked for v8M

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

We can freely add more items to vmstate_m_security without
breaking migration compatibility, because no CPU currently
has the ARM_FEATURE_M_SECURITY bit enabled and so this
subsection is not yet used by anything.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1503414539-28762-14-git-send-email-peter.maydell@linaro.org
