KVM: arm64: PMU: Add counter_index_to_*reg() helpers

In order to reduce the boilerplate code, add two helpers returning
the counter register index (resp. the event register) in the vcpu
register file from the counter index.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221113163832.3154370-8-maz@kernel.org