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>

diff --cc drivers/clk/clk.c
Simple merge

diff --cc include/linux/clk.h
Simple merge