powerpc/pseries/ras: Avoid calling rtas_token() in NMI paths

In the interest of reducing code and possible failures in the
machine check and system reset paths, grab the "ibm,nmi-interlock"
token at init time.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200508043408.886394-6-npiggin@gmail.com