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_t35_defconfig b/configs/cm_t35_defconfig
index f1fe2d0..a27c502 100644
--- a/configs/cm_t35_defconfig
+++ b/configs/cm_t35_defconfig
@@ -56,6 +56,7 @@
 CONFIG_USB_OMAP3=y
 CONFIG_TWL4030_USB=y
 CONFIG_USB_STORAGE=y
+CONFIG_USB_GADGET=y
 CONFIG_VIDEO_OMAP3=y
 CONFIG_LCD=y
 CONFIG_OF_LIBFDT=y