powerpc/pseries: Only call start-cpu when a CPU is stopped

Currently we always call start-cpu irrespective of if the CPU is
stopped or not. Unfortunatley on POWER7, firmware seems to not like
start-cpu being called when a cpu already been started.  This was not
the case on POWER6 and earlier.

This patch checks to see if the CPU is stopped or not via an
query-cpu-stopped-state call, and only calls start-cpu on CPUs which
are stopped.

This fixes a bug with kexec on POWER7 on PHYP where only the primary
thread would make it to the second kernel.

Reported-by: Ankita Garg <ankita@linux.vnet.ibm.com>
Cc: <stable@kernel.org>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>