i2c: Rename CONFIG_I2C_[0..5]_NAME -> DT_I2C_[0..5]_NAME

Now that everything is DT based for I2C drivers we can rename the
CONFIG_I2C_[0..5]_NAME define to DT_I2C_[0..5]_NAME.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>