ZONE_DEVICE and pmem API support for powerpc

Hi all,

This series adds support for ZONE_DEVICE and the pmem api on powerpc. Namely,
support for altmaps and the various bits and pieces required for DAX PMD faults.
The first two patches touch generic mm/ code, but otherwise this is fairly well
contained in arch/powerpc.

If the nvdimm folks could sanity check this series I'd appreciate it.

Series is based on next-20170411, but it should apply elsewhere with minor
fixups to arch_{add|remove}_memory due to conflicts with HMM.  For those
interested in testing this, there is a driver and matching firmware that carves
out some system memory for use as an emulated Con Tutto memory card.

Driver: https://github.com/oohal/linux/tree/contutto-next
Firmware: https://github.com/oohal/skiboot/tree/fake-contutto

Edit core/init.c:686 to control the amount of memory borrowed for the emulated
device.  I'm keeping the driver out of tree for a until 4.13 since I plan on
reworking the firmware interface anyway and There's at least one showstopper
bug.


Thanks,
Oliver