armv7m: QOMify the armv7m container

Create a proper QOM object for the armv7m container, which
holds the CPU, the NVIC and the bitband regions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1487604965-23220-4-git-send-email-peter.maydell@linaro.org