doc: update struct references to use :c:struct:

Sphinx>=3.0 includes the `struct` role in the C domain, which provides a
specific way to link to structs, while the old :c:type: role should be
primary used to typedefs. Update existing references, using :c:type:,
:cpp:type: or emphasized symbols that point to structs to use the new
role.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>