usb: host: dwc2: add clk support

Add support for clock with driver model.

This patch don't added dependency because when CONFIG_CLK
is not activated the clk function are stubbed.

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