doc: add include/dm/uclass.h to the HTML documentation

Correct Sphinx style comments in include/dm/uclass.h
and add the driver model UCLASS API to the HTML documentation.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/api/index.rst b/doc/api/index.rst
index 806c738..3f36174 100644
--- a/doc/api/index.rst
+++ b/doc/api/index.rst
@@ -7,6 +7,7 @@
    :maxdepth: 2
 
    dfu
+   dm
    efi
    getopt
    linker_lists