armv7m: Make NVIC expose a memory region rather than mapping itself

Make the NVIC device expose a memory region for its users
to map, rather than mapping itself into the system memory
space on realize, and get the one user (the ARMv7M object)
to do this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1487604965-23220-7-git-send-email-peter.maydell@linaro.org