doc: driver-model: Convert of-plat.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 ff9b183..d1c19a4 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -12,3 +12,4 @@
    i2c-howto
    livetree
    migration
+   of-plat