clk: Improve debug message in clk_set_default_rates()

It is helpful to print the clock number as well as the index, so that this
can be looked up in the binding file. Update the debug() statement to do
this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>