powerpc: Add reboot notifier to Collaborative Memory Manager

When running Active Memory Sharing, pages can get marked as
"loaned" with the hypervisor by the CMM driver. This state gets
cleared by the system firmware when rebooting the partition.
When using kexec to boot a new kernel, this state never gets
cleared and the hypervisor and CMM driver can get out of sync
with respect to the number of pages currently marked "loaned".
Fix this by adding a reboot notifier to the CMM driver to deflate
the balloon and mark all pages as active.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>