powerpc/powernv: Fix starting of secondary CPUs on OPALv2 and v3

The current code fails to handle kexec on OPALv2. This fixes it
and adds code to improve the situation on OPALv3 where we can
query the CPU status from the firmware and decide what to do
based on that.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>