powerpc: Move sparse_init() into initmem_init

We did part of sparse initialisation in setup_arch and part in
initmem_init. Put them together.

Signed-off-by: Anton Blanchard <anton@samba.org>
Tested-by: Emil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>