ASoC: stm32: fix dtbs_check warnings

Merge series from Olivier Moysan <olivier.moysan@foss.st.com>:

Fix dtbs_check warnings in STM32MP15 DK boards Devices Trees for
STM32 I2S and Cirrus CS42L51 codec.

- Add OF graph port property in I2S and CS42L51 DT bindings.
  Fixes warnings:
  audio-controller@4000b000: Unevaluated properties are not allowed
  ('port' was unexpected)
  cs42l51@4a: Unevaluated properties are not allowed
  ('port' was unexpected)
- Correct OF graph DAI audio format property for STM32MP15x Dkx I2S node