clk: add clk_round_rate()

It returns the rate which will be set if you ask clk_set_rate() to set
that rate. It provides a way to query exactly what rate you'll get if
you call clk_set_rate() with that same argument.
So essentially, clk_round_rate() and clk_set_rate() are equivalent
except the former does not modify the clock hardware in any way.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sean Anderson <seanga2@gmail.com>

include/clk-uclass.h

@@ -61,6 +61,14 @@ struct clk_ops {
 	 * @return 0 if OK, or a negative error code.
 	 */
 	int (*rfree)(struct clk *clock);
+	/**
+	 * round_rate() - Adjust a rate to the exact rate a clock can provide.
+	 *
+	 * @clk:	The clock to manipulate.
+	 * @rate:	Desidered clock rate in Hz.
+	 * @return rounded rate in Hz, or -ve error code.
+	 */
+	ulong (*round_rate)(struct clk *clk, ulong rate);
 	/**
 	 * get_rate() - Get current clock rate.
 	 *