dm: Add timeline and guide for porting I2C drivers

Add a README with a brief guide to porting i2c drivers over to use driver
model.

Add a timeline also. All I2C drivers should be converted by the end
of June 2017.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>

diff --git a/doc/driver-model/i2c-howto.txt b/doc/driver-model/i2c-howto.txt
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+How to port a serial driver to driver model
+===========================================
+
+Over half of the I2C drivers have been converted as at November 2016. These
+ones remain:
+
+   adi_i2c
+   davinci_i2c
+   fti2c010
+   ihs_i2c
+   kona_i2c
+   lpc32xx_i2c
+   pca9564_i2c
+   ppc4xx_i2c
+   rcar_i2c
+   sh_i2c
+   sh_sh7734_i2c
+   soft_i2c
+   tsi108_i2c
+   zynq_i2c
+
+The deadline for this work is the end of June 2017. If no one steps
+forward to convert these, at some point there may come a patch to remove them!
+
+Here is a suggested approach for converting your I2C driver over to driver
+model. Please feel free to update this file with your ideas and suggestions.
+
+- #ifdef out all your own I2C driver code (#ifndef CONFIG_DM_I2C)
+- Define CONFIG_DM_I2C 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 work fully since there will be
+    no I2C driver
+- Add the U_BOOT_DRIVER piece at the end (e.g. copy tegra_i2c.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