clk: Add client API to HTML docs

This converts the existing client (aka clk.h) documentation to kernel doc
format, and adds it to the HTML docs. I have tried to preserve existing
comments as much as possible, refraining from semantic changes.

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/20211222171114.3091780-4-seanga2@gmail.com


[rebased onto u-boot/master and resolved conflicts]
Signed-off-by: Sean Anderson <seanga2@gmail.com>