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: Ɓukasz Majewski <lukma@denx.de>
diff --git a/drivers/usb/gadget/Makefile b/drivers/usb/gadget/Makefile
index 5e316a7..6a007d1 100644
--- a/drivers/usb/gadget/Makefile
+++ b/drivers/usb/gadget/Makefile
@@ -28,6 +28,7 @@
 obj-$(CONFIG_USB_FUNCTION_DFU) += f_dfu.o
 obj-$(CONFIG_USB_FUNCTION_MASS_STORAGE) += f_mass_storage.o
 obj-$(CONFIG_USB_FUNCTION_FASTBOOT) += f_fastboot.o
+obj-$(CONFIG_USB_FUNCTION_SDP) += f_sdp.o
 endif
 endif
 ifdef CONFIG_USB_ETHER