clk: Use dummy clk_get_by_* functions when CONFIG_CLK is disabled

The implementations of clk_get_by_index & clk_get_by_name are only
available when CONFIG_CLK is enabled. Provide the dummies when this is
not the case in order to avoid build failures.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/clk.h

@@ -59,7 +59,7 @@ struct clk {
 	unsigned long id;
 };
 
-#if CONFIG_IS_ENABLED(OF_CONTROL)
+#if CONFIG_IS_ENABLED(OF_CONTROL) && CONFIG_IS_ENABLED(CLK)
 struct phandle_2_cell;
 int clk_get_by_index_platdata(struct udevice *dev, int index,
 			      struct phandle_2_cell *cells, struct clk *clk);