debugcc: allow leaf muxes to override measurement function

MCCC (memory clock controller) uses non-standard measuring process (just
read the rate and return). Provide a way to override default
measurement proces with the leaf cc hook.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

debugcc.c

@@ -195,7 +195,10 @@ static void measure(const struct measure_clk *clk)
 
 	mux_prepare_enable(clk->primary, clk->mux);
 
-	clk_rate = measure_default(clk);
+	if (clk->leaf && clk->leaf->measure)
+		clk_rate = clk->leaf->measure(clk);
+	else
+		clk_rate = measure_default(clk);
 
 	mux_disable(clk->primary);