ARM: dts: stm32: add uart7 support to stm32mp15xx-dkx boards

Adds uart7 node to stm32mp15xx-dkx and uart7 alias to stm32mp157a-dk1 and
stm32mp157c-dk2 boards. uart7 pins are connected to Arduino connector.
uart7 is disabled by default.
Signed-off-by: NErwan Le Ray <erwan.leray@st.com>
Signed-off-by: NAlexandre Torgue <alexandre.torgue@st.com>

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

@@ -19,6 +19,7 @@
 		ethernet0 = &ethernet0;
 		serial0 = &uart4;
 		serial1 = &usart3;
+		serial2 = &uart7;
 	};
 
 	chosen {

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

@@ -20,6 +20,7 @@
 		ethernet0 = &ethernet0;
 		serial0 = &uart4;
 		serial1 = &usart3;
+		serial2 = &uart7;
 	};
 
 	chosen {

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

@@ -591,6 +591,14 @@
 	status = "okay";
 };
 
+&uart7 {
+	pinctrl-names = "default", "sleep", "idle";
+	pinctrl-0 = <&uart7_pins_c>;
+	pinctrl-1 = <&uart7_sleep_pins_c>;
+	pinctrl-2 = <&uart7_idle_pins_c>;
+	status = "disabled";
+};
+
 &usart3 {
 	pinctrl-names = "default", "sleep", "idle";
 	pinctrl-0 = <&usart3_pins_c>;