arm: Use different ARMMMUIdx values for M profile

Make M profile use completely separate ARMMMUIdx values from
those that A profile CPUs use. This is a prelude to adding
support for the MPU and for v8M, which together will require
6 MMU indexes which don't map cleanly onto the A profile
uses:
 non secure User
 non secure Privileged
 non secure Privileged, execution priority < 0
 secure User
 secure Privileged
 secure Privileged, execution priority < 0

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1493122030-32191-4-git-send-email-peter.maydell@linaro.org
