memory hotplug: Define memory_block_size_bytes for powerpc/pseries

Define a version of memory_block_size_bytes() for powerpc/pseries such that
a memory block spans an entire lmb.

Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Reviewed-by: Robin Holt <holt@sgi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>