usb: Make compiling gadget support optional

There is no need to compile and include this code if it is not used.
CONFIG_USB_GADGET can be used for the purpose.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/configs/cm_t3517_defconfig b/configs/cm_t3517_defconfig
index 0901fea..c204a29 100644
--- a/configs/cm_t3517_defconfig
+++ b/configs/cm_t3517_defconfig
@@ -54,6 +54,7 @@
 CONFIG_USB_MUSB_HOST=y
 CONFIG_USB_MUSB_AM35X=y
 CONFIG_USB_STORAGE=y
+CONFIG_USB_GADGET=y
 CONFIG_VIDEO_OMAP3=y
 CONFIG_LCD=y
 CONFIG_OF_LIBFDT=y