hw/ppc/ppc405: Use memory_region_init_rom() with read-only regions

hw/ppc/ppc405: Use memory_region_init_rom() with read-only regions

The scripts/coccinelle/memory-region-housekeeping.cocci reported:
* TODO [[view:./hw/ppc/ppc405_boards.c::face=ovl-face1::linb=195::colb=8::cole=30][potential use of memory_region_init_rom*() in  ./hw/ppc/ppc405_boards.c::195]]
* TODO [[view:./hw/ppc/ppc405_boards.c::face=ovl-face1::linb=464::colb=8::cole=30][potential use of memory_region_init_rom*() in  ./hw/ppc/ppc405_boards.c::464]]

We can indeed replace the memory_region_init_ram() and
memory_region_set_readonly() calls by memory_region_init_rom().

Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>