target/arm: Enable MVE in Cortex-M55

We now have a complete MVE emulation, so we can enable it in our
Cortex-M55 model by setting the ID registers to match those of a
Cortex-M55 with full MVE support.

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