spapr: Move memory hotplug to RTAS_LOG_V6_HP_ID_DRC_COUNT type

Till now memory hotplug used RTAS_LOG_V6_HP_ID_DRC_INDEX hotplug type
which meant that we generated one hotplug type of EPOW event for every
256MB (SPAPR_MEMORY_BLOCK_SIZE). This quickly overruns the kernel
rtas log buffer thus resulting in loss of memory hotplug events. Switch
to RTAS_LOG_V6_HP_ID_DRC_COUNT hotplug type for memory so that we
generate only one event per hotplug request.

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