linux-4.9: Fix building of DWC3 module

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------------------

This was affecting Lede in case of CONFIG_ALL_KMODS=y.

The patch is already scheduled for v4.12 and once it is backported to
4.9.y it must be removed from here.

Note this patch makes sense not only for ARC instead it fixes
a generic issue which may affect more arches and platforms thus
putting it in "generic/" folder.

Signed-off-by: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [patch rename]