ARM: dts: stm32: Add idle/sleep pinmux for USART3 on stm32mp15

Add idle and sleep mux option for the USART3 pins_a.

This is used on the Phycore STM32MP1.
Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

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

@@ -2402,6 +2402,23 @@ pins2 {
 		};
 	};
 
+	usart3_idle_pins_a: usart3-idle-0 {
+		pins1 {
+			pinmux = <STM32_PINMUX('B', 10, ANALOG)>; /* USART3_TX */
+		};
+		pins2 {
+			pinmux = <STM32_PINMUX('B', 12, AF8)>; /* USART3_RX */
+			bias-disable;
+		};
+	};
+
+	usart3_sleep_pins_a: usart3-sleep-0 {
+		pins {
+			pinmux = <STM32_PINMUX('B', 10, ANALOG)>, /* USART3_TX */
+				 <STM32_PINMUX('B', 12, ANALOG)>; /* USART3_RX */
+		};
+	};
+
 	usart3_pins_b: usart3-1 {
 		pins1 {
 			pinmux = <STM32_PINMUX('B', 10, AF7)>, /* USART3_TX */