We always expose the HW view of PMU in ID_AA64FDR0_EL1.PMUver,
even when the PMU feature is disabled, while the architecture
says that FEAT_PMUv3 not being implemented should result in this
field being zero.

Let's follow the architecture's guidance.
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>