doc: driver-model: Convert fdt-fixup.txt to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/driver-model/index.rst b/doc/driver-model/index.rst
index 663aa5d..71ad062 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -7,4 +7,5 @@
    :maxdepth: 2
 
    design
+   fdt-fixup
    migration