math64: add kernel-doc for DIV64_U64_ROUND_UP

Add kernel-doc for DIV64_U64_ROUND_UP so that it appears in the
documentation.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20221118182309.3824530-2-liambeguin@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>