of: clk: Make <linux/of_clk.h> self-contained

Depending on include order:

    include/linux/of_clk.h:11:45: warning: ‘struct device_node’ declared inside parameter list will not be visible outside of this definition or declaration
     unsigned int of_clk_get_parent_count(struct device_node *np);
						 ^~~~~~~~~~~
    include/linux/of_clk.h:12:43: warning: ‘struct device_node’ declared inside parameter list will not be visible outside of this definition or declaration
     const char *of_clk_get_parent_name(struct device_node *np, int index);
					       ^~~~~~~~~~~
    include/linux/of_clk.h:13:31: warning: ‘struct of_device_id’ declared inside parameter list will not be visible outside of this definition or declaration
     void of_clk_init(const struct of_device_id *matches);
				   ^~~~~~~~~~~~

Fix this by adding forward declarations for struct device_node and
struct of_device_id.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lkml.kernel.org/r/20200205194649.31309-1-geert+renesas@glider.be


Signed-off-by: Stephen Boyd <sboyd@kernel.org>