doc: driver-model: Convert i2c-howto.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 6b8f181..ce857a3 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -9,4 +9,5 @@
    design
    fdt-fixup
    fs_firmware_loader
+   i2c-howto
    migration