dm: i2c: Rename driver model I2C functions to permit compatibility

Add a dm_ prefix to driver model I2C functions so that we can keep the old
ones around.

This is a little unfortunate, but on reflection it is too difficult to
change the API. We can undo this rename when most boards and drivers are
converted to use driver model for I2C.

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