Indentation should be done with spaces, not with TABs, in the PPC subsystem

There are still some files in the QEMU PPC code base that use TABs for indentation instead of using spaces in these files here:

 hw/ppc/spapr_rtas.c
 hw/ppc/spapr.c
 hw/ppc/ppc440_bamboo.c
 include/hw/ppc/ppc.h

The TABs should be replaced, to make sure that we have a consistent coding style with an indentation of 4 spaces everywhere.