KVM: arm64: PMU: Simplify PMCR_EL0 reset handling

Resetting PMCR_EL0 is a pretty involved process that includes
poisoning some of the writable bits, just because we can.

It makes it hard to reason about about what gets configured,
and just resetting things to 0 seems like a much saner option.

Reduce reset_pmcr() to just preserving PMCR_EL0.N from the host,
and setting PMCR_EL0.LC if we don't support AArch32.

Signed-off-by: Marc Zyngier <maz@kernel.org>