[POWERPC] Add remove_memory() for 64-bit powerpc

Supply remove_memory() function for 64-bit powerpc.  This is still
not quite complete as it needs to do some more arch-specific stuff,
which will be added in a later patch.

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
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>