doc: driver-model: Convert spi-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: NBin Meng <bmeng.cn@gmail.com>

doc/driver-model/index.rst

@@ -17,3 +17,4 @@ Driver Model
    pmic-framework
    remoteproc-framework
    serial-howto
+   spi-howto