ARM: dts: stm32: add usart3 node to stm32mp15xx-dkx boards

Adds usart3 node to stm32mp15xx-dkx and usart3 alias to stm32mp157a-dk1
and stm32mp157c-dk2 boards. usart3 pins are connected to GPIO Expansion
connector. usart3 is disabled by default.
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp157a-dk1.dts

@@ -18,6 +18,7 @@ / {
 	aliases {
 		ethernet0 = &ethernet0;
 		serial0 = &uart4;
+		serial1 = &usart3;
 	};
 
 	chosen {

arch/arm/boot/dts/stm32mp157c-dk2.dts

@@ -19,6 +19,7 @@ / {
 	aliases {
 		ethernet0 = &ethernet0;
 		serial0 = &uart4;
+		serial1 = &usart3;
 	};
 
 	chosen {

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

@@ -591,6 +591,15 @@ &uart4 {
 	status = "okay";
 };
 
+&usart3 {
+	pinctrl-names = "default", "sleep", "idle";
+	pinctrl-0 = <&usart3_pins_c>;
+	pinctrl-1 = <&usart3_sleep_pins_c>;
+	pinctrl-2 = <&usart3_idle_pins_c>;
+	uart-has-rtscts;
+	status = "disabled";
+};
+
 &usbh_ehci {
 	phys = <&usbphyc_port0>;
 	status = "okay";