target/arm: Make Cortex-M3 and M4 default to 8 PMSA regions

The Cortex-M3 and M4 CPUs always have 8 PMSA MPU regions (this isn't
a configurable option for the hardware).  Make the default value of
the pmsav7-dregion property be set per-cpu, so we don't need to have
every user of these CPUs set it manually.  (The existing default of
16 is correct for the other PMSAv7 core, the Cortex-R5.)

This fixes a bug where we were creating the M3 and M4 with
too many regions; most guest software would not notice or
care, though, since it would just not use the registers
associated with the unexpected extra regions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1499788408-10096-4-git-send-email-peter.maydell@linaro.org
