usb: host: dwc2: add phy support

Use generic phy to initialize the PHY associated to the
DWC2 device and available in the device tree.

This patch don't added dependency because when CONFIG_PHY
is not activated, the generic PHY function are stubbed.

Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>