hw/ppc/e500: Reduce usage of sysbus API

PlatformBusDevice has an mmio attribute which gets aliased to
SysBusDevice::mmio[0]. So PlatformbusDevice::mmio can be used directly,
avoiding the sysbus API.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20221003203142.24355-5-shentey@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

hw/ppc/e500.c

@@ -1023,7 +1023,7 @@ void ppce500_init(MachineState *machine)
 
         memory_region_add_subregion(address_space_mem,
                                     pmc->platform_bus_base,
-                                    sysbus_mmio_get_region(s, 0));
+                                    &pms->pbus_dev->mmio);
     }
 
     /*