powerpc/64s: POWER8 add missing machine check definitions

POWER8 uses bit 36 in SRR1 like POWER9 for i-side machine checks, and
contains several conditions for link timeouts that are not currently
handled.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>