arm64: KVM: Add temporary kvm_perf_event.h

In order to merge the KVM/ARM PMU patches without creating a
conflict mess, let's have a temporary include file that won't
conflict with anything. Subsequent patches will clean that up.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>