clk: add pr_debug & kerneldoc around clk notifiers

Both the pr_err and the additional kerneldoc aim to help when debugging
errors thrown from within a clock rate-change notifier callback.

Reported-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

include/linux/clk.h

@@ -78,8 +78,22 @@ struct clk_notifier_data {
 	unsigned long		new_rate;
 };
 
+/**
+ * clk_notifier_register: register a clock rate-change notifier callback
+ * @clk: clock whose rate we are interested in
+ * @nb: notifier block with callback function pointer
+ *
+ * ProTip: debugging across notifier chains can be frustrating. Make sure that
+ * your notifier callback function prints a nice big warning in case of
+ * failure.
+ */
 int clk_notifier_register(struct clk *clk, struct notifier_block *nb);
 
+/**
+ * clk_notifier_unregister: unregister a clock rate-change notifier callback
+ * @clk: clock whose rate we are no longer interested in
+ * @nb: notifier block which will be unregistered
+ */
 int clk_notifier_unregister(struct clk *clk, struct notifier_block *nb);
 
 /**