dm: Add timeline and guide for porting serial drivers

Add a README with a brief guide to porting serial drivers over to use
driver model.

Add a timeline also. All serial drivers should be converted by the end
of January 2016.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/driver-model/serial-howto.txt

+How to port a serial driver to driver model
+===========================================
+
+About 16 of 33 serial drivers have been converted as at September 2015. It
+is time for maintainers to start converting over the remaining serial drivers:
+
+   altera_jtag_uart.c
+   altera_uart.c
+   arm_dcc.c
+   lpc32xx_hsuart.c
+   mcfuart.c
+   mxs_auart.c
+   opencores_yanu.c
+   serial_bfin.c
+   serial_imx.c
+   serial_lpuart.c
+   serial_max3100.c
+   serial_pxa.c
+   serial_s3c24x0.c
+   serial_sa1100.c
+   serial_stm32.c
+   serial_xuartlite.c
+   usbtty.c
+
+You should complete this by the end of January 2016.
+
+Here is a suggested approach for converting your serial driver over to driver
+model. Please feel free to update this file with your ideas and suggestions.
+
+- #ifdef out all your own serial driver code (#ifndef CONFIG_DM_SERIAL)
+- 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
+- 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
+- If you convert all existing users of the driver, remove the pre-driver-model
+    code
+
+In terms of patches a conversion series typically has these patches:
+- clean up / prepare the driver for conversion
+- add driver model code
+- convert at least one existing board to use driver model serial
+- (if no boards remain that don't use driver model) remove the old code
+
+This may be a good time to move your board to use device tree also. Mostly
+this involves these steps:
+
+- define CONFIG_OF_CONTROL and CONFIG_OF_SEPARATE
+- add your device tree files to arch/<arch>/dts
+- update the Makefile there
+- Add stdout-path to your /chosen device tree node if it is not already there
+- build and get u-boot-dtb.bin so you can test it
+- Your drivers can now use device tree
+- For device tree in SPL, define CONFIG_SPL_OF_CONTROL