dm: i2c: Make API accessible even without CONFIG_DM

Make the driver model I2C API available always, even if driver model
is not enabled. This allows for a 'soft' switch-over, where drivers can
use the new structures in code which is compiled but not yet used. This
makes migration easier in some cases.

Fix up the existing drivers which define their own 'struct i2c_msg'.

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