test: dm: add test for phandle access functions

Add unitary test for phandle access functions
- ofnode_count_phandle_with_args
- ofnode_parse_phandle_with_args
- dev_count_phandle_with_args
- dev_read_phandle_with_args

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>