doc: spl: update reference to driver-model/README.txt

Update the reference in doc/README.SPL to a no more existing file:
"driver-model/README.txt", it is changed to "doc/driver-model/design.rst".

Adding the directory path /doc/ allows to check this reference with
'make refcheckdocs'.

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