KVM: arm/arm64: Show halt poll counters in debugfs

ARM/ARM64 has counters halt_successful_poll, halt_attempted_poll,
halt_poll_invalid, and halt_wakeup but never exposed those in debugfs.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1572164390-5851-1-git-send-email-borntraeger@de.ibm.com