target/arm: Implement Cortex-M55 model

Now that we have implemented all the features needed by the v8.1M
architecture, we can add the model of the Cortex-M55.  This is the
configuration without MVE support; we'll add MVE later.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201210201433.26262-5-peter.maydell@linaro.org