powerpc/powernv: Set memory_block_size_bytes to 256MB

powerpc sets a low SECTION_SIZE_BITS to accomodate small pseries
boxes. We default to 16MB memory blocks, and boxes with a lot
of memory end up with enormous numbers of sysfs memory nodes.

Set a more reasonable default for powernv of 256MB.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>