math64.h: kernel-docs: Convert some markups into normal comments

There are several functions at math64.h that are also
defined at div64.c. As both are included at kernel-api.rst,
Sphinx 3.x complains about symbol duplication:

	./lib/math/div64.c:73: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'div_s64_rem'.
	./lib/math/div64.c:104: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'div64_u64_rem'.
	./lib/math/div64.c:144: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'div64_u64'.
	./lib/math/div64.c:172: WARNING: Duplicate C declaration, also defined in 'core-api/kernel-api'.
	Declaration is 'div64_s64'.

In order to avoid Sphinx warnings about duplication, change the kernel-doc
markups to just comments at math64.h.

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>