armv7m: Implement M profile default memory map

armv7m: Implement M profile default memory map

Add support for the M profile default memory map which is used
if the MPU is not present or disabled.

The main differences in behaviour from implementing this
correctly are that we set the PAGE_EXEC attribute on
the right regions of memory, such that device regions
are not executable.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Message-id: 1493122030-32191-10-git-send-email-peter.maydell@linaro.org
[PMM: rephrased comment and commit message; don't mark
 the flash memory region as not-writable; list all
 the cases in the default map explicitly rather than
 using a 'default' case for the non-executable regions]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>