dm: Fix documentation entry as there is no UCLASS_CLOCK uclass

There is no UCLASS_CLOCK uclass defined. Instead we do use the UCLASS_CLK.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>