usb: gadget: add SDP driver

Add SDP (Serial Downloader Protocol) implementation for U-Boot. The
protocol is used in NXP SoC's boot ROM and allows to download program
images. Beside that, it can also be used to read/write registers and
download complete Device Configuration Data (DCD) sets. This basic
implementation supports downloading images with the imx header format
reading and writing registers.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Łukasz Majewski <lukma@denx.de>

include/sdp.h

@@ -0,0 +1,16 @@
+/*
+ * sdp.h - Serial Download Protocol
+ *
+ * Copyright (C) 2017 Toradex
+ * Author: Stefan Agner <stefan.agner@toradex.com>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef __SDP_H_
+#define __SDP_H_
+
+int sdp_init(int controller_index);
+void sdp_handle(int controller_index);
+
+#endif /* __SDP_H_ */