doc: driver-model: Convert serial-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>

doc/driver-model/index.rst

@@ -16,3 +16,4 @@ Driver Model
    pci-info
    pmic-framework
    remoteproc-framework
+   serial-howto

doc/driver-model/serial-howto.rst

@@ -1,11 +1,13 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
 How to port a serial driver to driver model
 ===========================================
 
 Almost all of the serial drivers have been converted as at January 2016. These
 ones remain:
 
-   serial_bfin.c
-   serial_pxa.c
+   * serial_bfin.c
+   * serial_pxa.c
 
 The deadline for this work was the end of January 2016. If no one steps
 forward to convert these, at some point there may come a patch to remove them!