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>