ARM: DT: STM32: add dma for usart3 on F429

Add DMA support for USART3 on STM32F429 MCU.
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32f429.dtsi

@@ -122,6 +122,9 @@ usart3: serial@40004800 {
 			interrupts = <39>;
 			clocks = <&rcc 0 146>;
 			status = "disabled";
+			dmas = <&dma1 1 4 0x400 0x0>,
+			       <&dma1 3 4 0x400 0x0>;
+			dma-names = "rx", "tx";
 		};
 
 		usart4: serial@40004c00 {