of: clk: Make of_clk_get_parent_{count,name}() parameter const

of_clk_get_parent_count() and of_clk_get_parent_name() never modify the
device nodes passed, so they can be const.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lkml.kernel.org/r/20200212094317.1150-1-geert+renesas@glider.be
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

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>

clk: Extract OF clock helpers in <linux/of_clk.h>

The use of of_clk_get_parent_{count,name}() and of_clk_init() is not
limited to clock providers.

Hence move these helpers into their own header file, so callers that are
not clock providers no longer have to include <linux/clk-provider.h>.

Suggested-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>