target/arm: Add M profile secure MMU index values to get_a32_user_mem_index()

Add the M profile secure MMU index values to the switch in
get_a32_user_mem_index() so that LDRT/STRT work correctly
rather than asserting at translate time.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1507556919-24992-2-git-send-email-peter.maydell@linaro.org