ppc/ppc405_boards: use memdev for RAM

ppc/ppc405_boards: use memdev for RAM

memory_region_allocate_system_memory() API is going away, so
replace it with memdev allocated MemoryRegion. The later is
initialized by generic code, so board only needs to opt in
to memdev scheme by providing
  MachineClass::default_ram_id
and using MachineState::ram instead of manually initializing
RAM memory region.

PS:
in ref405ep alias RAM into ram_memories[] to avoid re-factoring
its user ppc405ep_init(), which would be invasive and out of
scope this patch.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200219160953.13771-65-imammedo@redhat.com>