spapr: Add LMB DR connectors

Enable memory hotplug for pseries 2.4 and add LMB DR connectors.
With memory hotplug, enforce RAM size, NUMA node memory size and maxmem
to be a multiple of SPAPR_MEMORY_BLOCK_SIZE (256M) since that's the
granularity in which LMBs are represented and hot-added.

LMB DR connectors will be used by the memory hotplug code.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
               [spapr_drc_reset implementation]
[since this missed the 2.4 cutoff, changing to only enable for 2.5]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>