powerpc/pseries: Add CPU dlpar remove functionality

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

To remove multiple cpus in a single request we create a list of possible
DR (Dynamic Reconfiguration) cpus and their drc indexes that can be
removed.  We can then traverse the list remove each cpu and easily clean
up in any cases of failure.

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