doc: driver-model: Convert pmic-framework.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 a83c648..fd33215 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -14,3 +14,4 @@
    migration
    of-plat
    pci-info
+   pmic-framework