powerpc/pseries: Move smp_query_cpu_stopped() etc. out of plpar_wrappers.h

smp_query_cpu_stopped() and related #defines are currently in
plpar_wrappers.h. The function actually does an RTAS call, not an
hcall, and basically has nothing to do with plpar_wrappers.h

Move it into pseries.h, where it can easily be used by the only two
callers in pseries/smp.c and pseries/hotplug-cpu.c.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>