target/arm: Migrate MPU_RNR register state for M profile cores

The PMSAv7 region number register is migrated for R profile
cores using the cpreg scheme, but M profile doesn't use
cpregs, and so we weren't migrating the MPU_RNR register state
at all. Fix that by adding a migration subsection for the
M profile case.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1501153150-19984-6-git-send-email-peter.maydell@linaro.org