KVM: arm/arm64: Rename kvm_pmu_{enable/disable}_counter functions

The kvm_pmu_{enable/disable}_counter functions can enable/disable
multiple counters at once as they operate on a bitmask. Let's
make this clearer by renaming the function.

Suggested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Julien Thierry <julien.thierry@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>