spapr: Add event-scan and rtas-event-scan-rate to rtas node in DT

The rtasd kernel thread expects the presence of the event-scan rtas
call and rtas-event-scan-rate property in the DT /rtas node otherwise
the error_log is not created in /proc/ppc64/rtas preventing events
surfaced through check-exception from being recorded.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
spapr-pci: Fixup ibm,drc-* values and send slot drc_index during hotplug

Fixed off by one array indexing issues. DrcEntry->child_entries should start
at index 0. The ibm,drc-* properties have array size at index 0 but all name
and index calculations should use (i - 1) so that first value matches slot 0.
The "Slot XXX" drc_names for a PHB should be (bus * 32) + slot #.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>