[POWERPC] Update wait_state_cycles in the VPA

The hypervisor can look at the value in the wait_state_cycles field of
the VPA for an estimate of how busy dedicated processors are.
Currently, as the kernel never touches this field, we appear to be
100% busy.  This records the duration the kernel is in powersave and
passes that to the HV to provide a reasonable indication of
utilisation.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>