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