powerpc: Allow ppc_md platform hook to override memory_block_size_bytes

The pseries platform code unconditionally overrides
memory_block_size_bytes regardless of the running platform.

Create a ppc_md hook that so each platform can choose to
do what it wants.

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

arch/powerpc/include/asm/machdep.h

@@ -98,6 +98,9 @@ struct machdep_calls {
 	void		(*iommu_save)(void);
 	void		(*iommu_restore)(void);
 #endif
+#ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
+	unsigned long	(*memory_block_size)(void);
+#endif
 #endif /* CONFIG_PPC64 */
 
 	void		(*pci_dma_dev_setup)(struct pci_dev *dev);