ARM: dts: stm32: fix i2s endpoint format property for stm32mp15xx-dkx

Use "dai-format" to configure DAI audio format as specified in
audio-graph-port.yaml bindings.

Fixes: 144d1ba7 ("ARM: dts: stm32: Adapt STM32MP157 DK boards to stm32 DT diversity")
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>