ARM: dts: stm32: add usart2 node to stm32mp157c-dk2

Adds the usart2 node to stm32mp157c-dk2 board. usart2 pins are connected
to Bluetooth component. usart2 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/stm32mp157c-dk2.dts

@@ -21,6 +21,7 @@
 		serial0 = &uart4;
 		serial1 = &usart3;
 		serial2 = &uart7;
+		serial3 = &usart2;
 	};
 
 	chosen {
@@ -86,3 +87,11 @@
 		};
 	};
 };
+
+&usart2 {
+	pinctrl-names = "default", "sleep", "idle";
+	pinctrl-0 = <&usart2_pins_c>;
+	pinctrl-1 = <&usart2_sleep_pins_c>;
+	pinctrl-2 = <&usart2_idle_pins_c>;
+	status = "disabled";
+};