clk: Add driver API to HTML docs

This converts the existing driver API docs (clk-uclass.h) to kernel doc
format and adds them to the HTML documentation. Because the kernel doc
sphinx converter does not handle functions in structs very well, the
individual methods are documented separately. This is primarily inspired by
the phylink documentation [1], which uses this trick extensively.

[1] https://www.kernel.org/doc/html/latest/networking/kapi.html#c.phylink_mac_ops



Signed-off-by: Sean Anderson <seanga2@gmail.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20211222171114.3091780-5-seanga2@gmail.com