spapr: Support hotplug by specifying DRC count

Support hotplug identifier type RTAS_LOG_V6_HP_ID_DRC_COUNT that allows
hotplugging of DRCs by specifying the DRC count.

While we are here, rename

spapr_hotplug_req_add_event() to spapr_hotplug_req_add_by_index()
spapr_hotplug_req_remove_event() to spapr_hotplug_req_remove_by_index()

so that they match with spapr_hotplug_req_add_by_count().

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>