powerpc/pseries: Implement memory hotplug remove in the kernel

This patch adds the ability to do memory hotplug remove in the kernel.

Currently the operation to hotplug remove memory is handled by the drmgr
command which performs the operation by performing some work in user-space
and making requests to the kernel to handle other pieces. By moving all
of the work to the kernel we can do the remove faster, and provide a common
code path to do memory hotplug for both the PowerVM and PowerKVM environments.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>