clk: also handle ENOENT in *_optional functions

If the device does not specify any clocks in device tree, these
functions will return PTR_ERR(-ENOENT). This is not the intended
behavior and does not comply with linux kernel CCF. Fix that by
returning NULL under such circumstances instead.

Signed-off-by: Yang Xiwen <forbidden405@outlook.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Link: https://lore.kernel.org/r/20230818-clk-fix-v1-3-49ec18f820bf@outlook.com