zephyr: CDC ACM UART node to specific overlays

Move CDC ACM UART node from common DTC overlay to
specific overlays, nrf52840_big.overlays and
usb_cdc_acm.overlays.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
diff --git a/boot/zephyr/boards/nrf52840_big.overlay b/boot/zephyr/boards/nrf52840_big.overlay
index 094a7a4..778a1ed 100644
--- a/boot/zephyr/boards/nrf52840_big.overlay
+++ b/boot/zephyr/boards/nrf52840_big.overlay
@@ -28,3 +28,10 @@
 			};
 	};
 };
+
+&zephyr_udc0 {
+	cdc_acm_uart0 {
+		compatible = "zephyr,cdc-acm-uart";
+		label = "CDC_ACM_0";
+	};
+};