boards: stm32: Convert I2S users to dt based pinctrl config

I2S signals should now be configured using dt.

For nucleo_f411re, I2S pins were configured using SPI definitions
that pulled the same strings behind the scene as I2S would have done.
This being said, only CK and SD pins should be needed and were
documented, so I only left those.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>