clk: hisi: refine parameter checking for init

*of_iomap()* will check the device node pointer, and if the pointer is
NULL it will return error code. So refine clock's init flow by checking
the device node with this simple way; and polish a little for the print
out message.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>