rockchip: clk: rk3399: handle clk_enable requests for USB3

The "simple" OF glue layer for the Designware USB3 core enables
all refernced clocks.  These need to be need to be implemented
otherwise the driver fails to probe.  A dummy implementation
that simply returns success is sufficient since the RK3399 comes
out of reset with all clock gates open.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>