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/usb_cdc_acm.overlay b/boot/zephyr/usb_cdc_acm.overlay
new file mode 100644
index 0000000..09eb8cf
--- /dev/null
+++ b/boot/zephyr/usb_cdc_acm.overlay
@@ -0,0 +1,6 @@
+&zephyr_udc0 {
+	cdc_acm_uart0 {
+		compatible = "zephyr,cdc-acm-uart";
+		label = "CDC_ACM_0";
+	};
+};