usb: dwc3: of-simple: remove include of clk-provider.h

The header clk-provider.h is there for clock drivers (providers) to
include, not client drivers (consumers).  That said,
of_clk_get_parent_count() is a helper function for clock providers, not
a clk API for consumers.

Let's replace of_clk_get_parent_count() with of_count_phandle_with_args()
call, so that we can remove the include of clk-provider.h.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>