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>

arch/arm/boot/dts/stm32mp15xx-dkx.dtsi

@@ -446,7 +446,7 @@ &i2s2 {
 	i2s2_port: port {
 		i2s2_endpoint: endpoint {
 			remote-endpoint = <&sii9022_tx_endpoint>;
-			format = "i2s";
+			dai-format = "i2s";
 			mclk-fs = <256>;
 		};
 	};