armv7m: Make ARMv7M object take memory region link

Make the ARMv7M object take a memory region link which it uses
to wire up the bitband rather than having them always put
themselves in the system address space.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1487604965-23220-6-git-send-email-peter.maydell@linaro.org