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>

diff --cc arch/powerpc/include/asm/machdep.h
Simple merge

diff --cc arch/powerpc/kernel/setup_64.c
Simple merge

diff --cc arch/powerpc/platforms/pseries/hotplug-memory.c
Simple merge

diff --cc arch/powerpc/platforms/pseries/pseries.h
Simple merge

diff --cc arch/powerpc/platforms/pseries/setup.c
Simple merge