ARM: dts: stm32: add usart3 node to stm32mp157c-ev1

Adds the usart3 node to stm32mp157c-ev1 board. 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/stm32mp157c-ev1.dts

@@ -19,6 +19,7 @@ chosen {
 
 	aliases {
 		serial0 = &uart4;
+		serial1 = &usart3;
 		ethernet0 = &ethernet0;
 	};
 
@@ -341,6 +342,20 @@ timer@11 {
 	};
 };
 
+&usart3 {
+	pinctrl-names = "default", "sleep", "idle";
+	pinctrl-0 = <&usart3_pins_b>;
+	pinctrl-1 = <&usart3_sleep_pins_b>;
+	pinctrl-2 = <&usart3_idle_pins_b>;
+	/*
+	 * HW flow control USART3_RTS is optional, and isn't default wired to
+	 * the connector. SB23 needs to be soldered in order to use it, and R77
+	 * (ETH_CLK) should be removed.
+	 */
+	uart-has-rtscts;
+	status = "disabled";
+};
+
 &usbh_ehci {
 	phys = <&usbphyc_port0>;
 	status = "okay";