powerpc/powernv: display reason for Malfunction Alert HMI.

The V2 version of HMI event now carries additional information for
Malfunction Alert. It now contains error information about CORE and NX
checkstop. This patch checks and displays the check stop reason before
panic.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>