Set the ID register bits to provide ARMv8.4-PMU (and implicitly
also ARMv8.1-PMU) in the 'max' CPU.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200214175116.9164-16-peter.maydell@linaro.org