cmd: clk: Add trivial implementation of clock dump for DM

Add trivial implementation of the clk dump in case DM is enabled.
This implementation just iterates over all the clock registered
with the CLK uclass and prints their rate.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <chin.liang.see@intel.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>