dt-bindings: Add DMA bindings for STM32 USART

Signed-off-by: Gerald Baeza <[e-mail]>
Signed-off-by: Alexandre TORGUE <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/serial/st,stm32-usart.txt

···
 Optional properties:
 - pinctrl: The reference on the pins configuration
 - st,hw-flow-ctrl: bool flag to enable hardware flow control.
+- dmas: phandle(s) to DMA controller node(s). Refer to stm32-dma.txt
+- dma-names: "rx" and/or "tx"
 
 Examples:
 usart4: serial@40004c00 {
···
 	st,hw-flow-ctrl;
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_usart2 &pinctrl_usart2_rtscts>;
+};
+
+usart1: serial@40011000 {
+	compatible = "st,stm32-usart", "st,stm32-uart";
+	reg = <0x40011000 0x400>;
+	interrupts = <37>;
+	clocks = <&rcc 0 164>;
+	dmas = <&dma2 2 4 0x414 0x0>,
+	       <&dma2 7 4 0x414 0x0>;
+	dma-names = "rx", "tx";
 };