KVM: arm64: Simplify handling of CNTKCTL_EL12

KVM: arm64: Simplify handling of CNTKCTL_EL12

We go trough a great deal of effort to map CNTKCTL_EL12 to CNTKCTL_EL1
while hidding this mapping from userspace via a special visibility helper.

However, it would be far simpler to just provide an accessor doing the
mapping job, removing the need for a visibility helper.

With that done, we can also remove the EL12_REG() macro which serves
no purpose.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240904082419.1982402-2-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>