powerpc/pseries: Partition migration in the kernel

Enable partition migration in the kernel.  To do this a new sysfs file,
/sys/kernel/mobility/migration, is created.  In order to initiate a migration
the stream id (generated by the HMC managing the system) is written to this
file.

After a migration occurs, and what is the majority of this code, the device
tree needs to be updated for the new system the partition is running on. This
is done via the ibm,update-nodes and ibm,update-properties rtas calls which
return information regarding which nodes and properties of the device tree
are to be added/removed/updated.

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