[POWERPC] Hotplug memory remove notifications for powerpc

Hotplug memory remove notifier for 64-bit powerpc.  This gets invoked
by writing to /proc/ppc64/ofdt the string "remove_node " followed by
the firmware device tree pathname of the node that needs to be removed.

In response, this adjusts the sections and removes sysfs entries by
calling __remove_pages().  Then it calls arch-specific code to get rid
of the hardware MMU mappings for the section of memory.

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Reviewed-by: Michael Ellerman <michael@ellerman.id.au>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>