spapr: Split memory nodes to power-of-two blocks

Linux kernel expects nodes to have power-of-two size and
does WARN_ON if this is not the case:
[    0.041456] WARNING: at drivers/base/memory.c:115
which is:

===
	/* Validate blk_sz is a power of 2 and not less than section size */
	if ((block_sz & (block_sz - 1)) || (block_sz < MIN_MEMORY_BLOCK_SIZE)) {
        	WARN_ON(1);
	        block_sz = MIN_MEMORY_BLOCK_SIZE;
	}
===

This splits memory nodes into set of smaller blocks with
a size which is a power of two. This makes sure the start
address of every node is aligned to the node size.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[agraf: squash windows compile fix in]
Signed-off-by: Alexander Graf <agraf@suse.de>
