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!
@@ -17,14 +19,14 @@ model. Please feel free to update this file with your ideas and suggestions.
 - Define CONFIG_DM_SERIAL for your board, vendor or architecture
 - If the board does not already use driver model, you need CONFIG_DM also
 - Your board should then build, but will not boot since there will be no serial
-    driver
+  driver
 - Add the U_BOOT_DRIVER piece at the end (e.g. copy serial_s5p.c for example)
 - Add a private struct for the driver data - avoid using static variables
 - Implement each of the driver methods, perhaps by calling your old methods
 - You may need to adjust the function parameters so that the old and new
-    implementations can share most of the existing code
+  implementations can share most of the existing code
 - If you convert all existing users of the driver, remove the pre-driver-model
-    code
+  code
 
 In terms of patches a conversion series typically has these patches:
 - clean up / prepare the driver for conversion