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>

drivers/clk/clk.c

@@ -1339,8 +1339,11 @@ static int __clk_speculate_rates(struct clk *clk, unsigned long parent_rate)
 	if (clk->notifier_count)
 		ret = __clk_notify(clk, PRE_RATE_CHANGE, clk->rate, new_rate);
 
-	if (ret & NOTIFY_STOP_MASK)
+	if (ret & NOTIFY_STOP_MASK) {
+		pr_debug("%s: clk notifier callback for clock %s aborted with error %d\n",
+				__func__, clk->name, ret);
 		goto out;
+	}
 
 	hlist_for_each_entry(child, &clk->children, child_node) {
 		ret = __clk_speculate_rates(child, new_rate);