Revert "target/arm: Make number of counters in PMCR follow the CPU"

This reverts commit f7fb73b8.

This change turned out to be a bit half-baked, and doesn't
work with KVM, which fails with the error:
   "qemu-system-aarch64: Failed to retrieve host CPU features"

because KVM does not allow accessing of the PMCR_EL0 value in
the scratch "query CPU ID registers" VM unless we have first
set the KVM_ARM_VCPU_PMU_V3 feature on the VM.

Revert the change for 6.0.

Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Zenghui Yu <yuzenghui@huawei.com>
Message-id: 20210331154822.23332-1-peter.maydell@linaro.org