Add OPAL log to collected file list

The OPAL console log used to live under /sys/kernel/debug/powerpc so it
was collected as part of that glob. This patch adds the new location at
/sys/firmware/opal/msglog to the list of collected files.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>