target/arm: Define an aa32_pmu_8_1 isar feature test function

target/arm: Define an aa32_pmu_8_1 isar feature test function

Instead of open-coding a check on the ID_DFR0 PerfMon ID register
field, create a standardly-named isar_feature for "does AArch32 have
a v8.1 PMUv3" and use it.

This entails moving the id_dfr0 field into the ARMISARegisters struct.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200214175116.9164-9-peter.maydell@linaro.org