KVM: arm64: PMU: Allow ID_DFR0_EL1.PerfMon to be set from userspace

Allow userspace to write ID_DFR0_EL1, on the condition that only
the PerfMon field can be altered and be something that is compatible
with what was computed for the AArch64 view of the guest.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221113163832.3154370-13-maz@kernel.org