ARM: dts: stm32: Add USART3 pins A pinmux entry on stm32mp1

Add pinmux for USART3.
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp15-pinctrl.dtsi

@@ -1054,6 +1054,19 @@ pins {
 		};
 	};
 
+	usart3_pins_a: usart3-0 {
+		pins1 {
+			pinmux = <STM32_PINMUX('B', 10, AF7)>; /* USART3_TX */
+			bias-disable;
+			drive-push-pull;
+			slew-rate = <0>;
+		};
+		pins2 {
+			pinmux = <STM32_PINMUX('B', 12, AF8)>; /* USART3_RX */
+			bias-disable;
+		};
+	};
+
 	uart4_pins_a: uart4-0 {
 		pins1 {
 			pinmux = <STM32_PINMUX('G', 11, AF6)>; /* UART4_TX */