usb: gadget: tegra-xudc: Add usb-phy support

usb-phy is used to get notified on the USB role changes. Get usb-phy from
the UTMI PHY.

Signed-off-by: Nagarjuna Kristam <nkristam@nvidia.com>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>