[POWERPC] Use H_CEDE on non-SMT

On the JS21 systems, they have the SPLPAR hypertas set, but are not SMT
capable.  So, they are not making the H_CEDE call.  This is causing the
hypervisor to have to queue up work for the hdecr, taking an excessive
amount of time in maintenance code, and causing jitter on the box.

Making the H_CEDE call helps alleviate that problem.

Signed-off-by: Jake Moilanen <moilanen@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>