powerpc/traps: add NOKPROBE_SYMBOL for sreset and mce

These NMIs could fire any time including inside kprobe code, so
exclude them from kprobes.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-19-npiggin@gmail.com