target/arm: Add dummy needed functions to M profile vmstate subsections

Currently the migration code incorrectly treats a subsection with
no .needed function pointer as if it was the subsection list
terminator -- it is ignored and so is everything after it.
Work around this by giving various M profile vmstate structs
a 'needed' function that always returns true.
We reuse m_needed() for this, since it's always true here.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180806123445.1459-4-peter.maydell@linaro.org