For memory remove, we need to clean up htab mappings for the
section of the memory we are removing.

This implements support for removing htab bolted mappings for pSeries
logical partitions.  Other sub-archs may need to implement similar
functionality for hotplug memory remove to work on them.
Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>