[doxygen] add doxygen comment for clk.c and clk.h

Accroding to RT-Thread#9947 and RT-Thread#9424, adding doxygen comment for function in .c files and data structure, macro in .h file is a solution.For this pr, I encountered problem following situations.
    - rt_clk_unprepare function return value type is different from rt_clk_ops so I change type of rt_clk_unprepare and its dependencies.
    - clk_get function may be similar as rt_clk_unprepare but I'm not sure so not modify.
    - clk_release implements may be incomplete, so I just marked in comment.

Signed-off-by: 1078249029 <[email protected]>

Author: 1078249029