Add rpi pico overlay file

Add rpi pico overlay file to the mtp sample

Author: ExtremeGTX

samples/subsys/usb/mtp/boards/rpi_pico.overlay

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2024 Kelly Lord
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&flash0 {
+	partitions {
+		compatible = "fixed-partitions";
+		#address-cells = <1>;
+		#size-cells = <1>;
+
+		storage_partition: partition@100000 {
+			label = "storage";
+			reg = <0x100000  DT_SIZE_M(1)>;
+		};
+	};
+};
+
+/ {
+	fstab {
+		compatible = "zephyr,fstab";
+		lfs1: lfs1 {
+			compatible = "zephyr,fstab,littlefs";
+			read-size = <32>;
+			prog-size = <32>;
+			cache-size = <256>;
+			lookahead-size = <64>;
+			block-cycles = <512>;
+			partition = <&storage_partition>;
+			mount-point = "/lfs1";
+			automount;
+			mtp-enabled;
+		};
+	};
+};