Forward declare struct of_device_id in linux/clk.h in order to avoid
warnings like the following:

warning: 'struct of_device_id' declared inside parameter list will not be visible outside
of this definition or declaration
 int of_clk_init(struct device_node *root, const struct of_device_id *matches);
                                                        ^~~~~~~~~~~~

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>