KVM: arm64: Let kvm_vcpu_read_pmcr() return an EL-dependent value for PMCR_EL0.N

When EL2 is present, PMCR_EL0.N is the effective value of MDCR_EL2.HPMN
when accessed from EL1 or EL0.

Make sure we honor this requirement.

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