usb: dwc3: Makefile: Make dwc3 driver compile in u-boot

Now that the entire dwc3 driver is adapted to compile with uboot build,
modify the Makefiles so that the dwc3 driver can be built.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
diff --git a/drivers/usb/gadget/udc/Makefile b/drivers/usb/gadget/udc/Makefile
index be265aa..12380f4 100644
--- a/drivers/usb/gadget/udc/Makefile
+++ b/drivers/usb/gadget/udc/Makefile
@@ -1,4 +1,4 @@
 #
 # USB peripheral controller drivers
 #
-obj-$(CONFIG_USB_GADGET)	+= udc-core.o
+obj-$(CONFIG_USB_DWC3_GADGET)	+= udc-core.o