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: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/driver-model/index.rst b/doc/driver-model/index.rst
index 82f4393..64151bd 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -17,3 +17,4 @@
    pmic-framework
    remoteproc-framework
    serial-howto
+   spi-howto