powerpc/nvram: Don't overwrite oops/panic report on normal shutdown

For normal halt, reboot, and poweroff events, refrain from overwriting
the lnx,oops-log partition.  Also, don't save the dmesg buffer on an
emergency-restart event if we've already saved it earlier in panic().

Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>