samples: dfu_multi_image: Align slot3 size

Change slot3_partition size to 256 KB to match the default network-core
primary slot.

Signed-off-by: Adam Szczygieł <adam.szczygiel@nordicsemi.no>

Ref: ncsdk-39942

Author: adsz-nordic

samples/dfu/dfu_multi_image/boards/nrf5340dk_nrf5340_cpuapp.overlay

@@ -39,10 +39,11 @@
 			reg = <0x44000 DT_SIZE_K(208)>;
 		};
 
+		/* 256 KB to match the default network-core primary slot (slot2/RAM flash). */
 		slot3_partition: partition@78000 {
 			compatible = "zephyr,mapped-partition";
 			label = "image-3";
-			reg = <0x78000 DT_SIZE_K(208)>;
+			reg = <0x78000 DT_SIZE_K(256)>;
 		};
 
 		dfu_multi_image_helper_partition: partition@b8000 {