powerpc/powernv: Show checkstop reason for NPU2 HMIs

If the kernel is notified of an HMI caused by the NPU2, it's currently
not being recognized and it logs the default message:

    Unknown Malfunction Alert of type 3

The NPU on Power 9 has 3 Fault Isolation Registers, so that's a lot of
possible causes, but we should at least log that it's an NPU problem
and report which FIR and which bit were raised if opal gave us the
information.

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>