usb: Make sure usb/phy/of gets built-in

DWC3 driver uses of_usb_get_phy_mode() which is
implemented in drivers/usb/phy/of.c and in bare minimal
configuration it might not be pulled in kernel binary.

In case of ARC or ARM this could be easily reproduced with
"allnodefconfig" +CONFIG_USB=m +CONFIG_USB_DWC3=m.

On building all ends-up with:
---------------------->8------------------
  Kernel: arch/arm/boot/Image is ready
  Kernel: arch/arm/boot/zImage is ready
  Building modules, stage 2.
  MODPOST 5 modules
ERROR: "of_usb_get_phy_mode" [drivers/usb/dwc3/dwc3.ko] undefined!
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2
---------------------->8------------------

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: linux-snps-arc@lists.infradead.org
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>