powerpc/powernv: Use the "unknown" checkstop type as a fallback

The HMI code knows about three types of errors: CORE, NX and UNKNOWN.
If OPAL were to add a new type, it would not be handled at all since
there is no fallback case.  Instead of explicitly checking for UNKNOWN,
treat any checkstop type without a handler as unknown.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>