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: NPatrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

doc/README.SPL

@@ -81,7 +81,7 @@ fdtgrep is also used to remove:
   ('u-boot,dm-pre-reloc', 'u-boot,dm-spl' and 'u-boot,dm-tpl')
 
 All the nodes remaining in the SPL devicetree are bound
-(see driver-model/README.txt).
+(see doc/driver-model/design.rst).
 
 Debugging
 ---------