ppc: rs6000_mc: drop usage of memory_region_allocate_system_memory()

ppc: rs6000_mc: drop usage of memory_region_allocate_system_memory()

rs6000mc_realize() violates memory_region_allocate_system_memory() contract
by calling it multiple times which could break -mem-path. Replace it with
plain memory_region_init_ram() instead.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20191008113318.7012-3-imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>