KVM: arm64: Don't let userspace write to PMCR_EL0.N when the vcpu has EL2

Now that userspace can provide its limit for hte maximum number of
counters, prevent it from writing to PMCR_EL0.N, as the value should
be derived from MDCR_EL2.HPMN in that case.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>