arm: Add support for M profile CPUs having different MMU index semantics

The M profile CPU's MPU has an awkward corner case which we
would like to implement with a different MMU index.

We can avoid having to bump the number of MMU modes ARM
uses, because some of our existing MMU indexes are only
used by non-M-profile CPUs, so we can borrow one.
To avoid that getting too confusing, clean up the code
to try to keep the two meanings of the index separate.

Instead of ARMMMUIdx enum values being identical to core QEMU
MMU index values, they are now the core index values with some
high bits set. Any particular CPU always uses the same high
bits (so eventually A profile cores and M profile cores will
use different bits). New functions arm_to_core_mmu_idx()
and core_to_arm_mmu_idx() convert between the two.

In general core index values are stored in 'int' types, and
ARM values are stored in ARMMMUIdx types.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1493122030-32191-3-git-send-email-peter.maydell@linaro.org

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