armv7m: Pass through start-powered-off CPU property

armv7m: Pass through start-powered-off CPU property

Expose "start-powered-off" as a property of the ARMv7M container,
which we just pass through to the CPU object in the same way that we
do for "init-svtor" and "idau". (We want this for the SSE-200, which
powers up only the first CPU at reset and leaves the second powered
down.)

As with the other CPU properties here, we can't just use alias
properties, because the CPU QOM object is not created until armv7m
realize time.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190121185118.18550-4-peter.maydell@linaro.org