KVM: PPC: e500: Emulate power management control SPR

For FSL e6500 core the kernel uses power management SPR register (PWRMGTCR0)
to enable idle power down for cores and devices by setting up the idle count
period at boot time. With the host already controlling the power management
configuration the guest could simply benefit from it, so emulate guest request
as a general store.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>