dm: i2c: Add a compatbility layer

For boards which use multiple I2C devices, or for SOCs which support
multiple boards, we might want to convert these to driver model at different
times. At present this is difficult because we need to either use
CONFIG_DM_I2C for a board or not.

Add a compatibility layer which implements the old API, thus allowing a
board to move to driver model for I2C without requiring that everything it
uses is moved in the same commit.

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