powerpc/64s: Invalidate ERAT on powersave wakeup for POWER9

On POWER9 the ERAT may be incorrect on wakeup from some stop states
that lose state. This causes random segvs and illegal instructions
when these stop states are enabled.

This patch invalidates the ERAT on wakeup on POWER9 to prevent this
from causing a problem.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Merge comment change with upstream changes]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>