usb: gadget: udc: renesas_usb3: add support for generic phy

This patch adds support for generic phy as an optional. If you want
to use a generic phy (e.g. phy-rcar-gen3-usb3 driver) on this driver,
you have to do "insmod phy-rcar-gen3-usb3.ko" first for now.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>