powerpc/pseries: Add CPU dlpar add functionality

Add the ability to hotplug add cpus via rtas hotplug events by either
specifying the drc index of the CPU to add, or providing a count of the
number of CPUs to add.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>