ARM: dts: stm32: fix a typo for DAC io-channel-cells on stm32mp15

Fix a typo on STM32MP15 DAC, e.g. s/channels/channel

Fixes: da6cddc7 ("ARM: dts: stm32: Add DAC support to stm32mp157c")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp151.dtsi

@@ -550,14 +550,14 @@ dac: dac@40017000 {
 
 			dac1: dac@1 {
 				compatible = "st,stm32-dac";
-				#io-channels-cells = <1>;
+				#io-channel-cells = <1>;
 				reg = <1>;
 				status = "disabled";
 			};
 
 			dac2: dac@2 {
 				compatible = "st,stm32-dac";
-				#io-channels-cells = <1>;
+				#io-channel-cells = <1>;
 				reg = <2>;
 				status = "disabled";
 			};