powernv: Clear SPRN_PSSCR when a POWER9 CPU comes online

Ensure that PSSCR is set to a safe value corresponding to no
state-loss each time a POWER9 CPU comes online.

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Acked-By: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>