Code that disables the i2c slave is now in the mvtwsi i2c driver.
Platform must enable DM_I2C to use that code. Add a comment in the code
as a reminder for the planned DM_I2C migration of Turris Omnia.

Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>