dt-bindings: Add DMA bindings for STM32 USART

Signed-off-by: NGerald Baeza <gerald.baeza@st.com>
Signed-off-by: NAlexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -12,6 +12,8 @@ stm32(f4) or stm32f7.
 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 {
@@ -32,3 +34,13 @@ usart2: serial@40004400 {
 	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";
+};