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>