KVM: arm64: Add minimal handling for the ARMv8.7 PMU

When running a KVM guest hosted on an ARMv8.7 machine, the host
kernel complains that it doesn't know about the architected number
of events.

Fix it by adding the PMUver code corresponding to PMUv3 for ARMv8.7.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211126115533.217903-1-maz@kernel.org