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>