PPC: KVM: set has-idle in guest device tree

On e500mc, the platform doesn't provide a way for the CPU to go idle.

To still not uselessly burn CPU time, expose an idle hypercall to the guest
if kvm supports it.

Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
[agraf: adjust for current code base, add patch description, fix non-kvm case]
Signed-off-by: Alexander Graf <agraf@suse.de>
