DOCUMENTATION: dt-bindings: Document the STM32 USART bindings

This adds documentation of device tree bindings for the
STM32 USART
Signed-off-by: NMaxime Coquelin <mcoquelin.stm32@gmail.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

+* STMicroelectronics STM32 USART
+
+Required properties:
+- compatible: Can be either "st,stm32-usart", "st,stm32-uart",
+"st,stm32f7-usart" or "st,stm32f7-uart" depending on whether
+the device supports synchronous mode and is compatible with
+stm32(f4) or stm32f7.
+- reg: The address and length of the peripheral registers space
+- interrupts: The interrupt line of the USART instance
+- clocks: The input clock of the USART instance
+
+Optional properties:
+- pinctrl: The reference on the pins configuration
+- st,hw-flow-ctrl: bool flag to enable hardware flow control.
+
+Examples:
+usart4: serial@40004c00 {
+	compatible = "st,stm32-uart";
+	reg = <0x40004c00 0x400>;
+	interrupts = <52>;
+	clocks = <&clk_pclk1>;
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_usart4>;
+};
+
+usart2: serial@40004400 {
+	compatible = "st,stm32-usart", "st,stm32-uart";
+	reg = <0x40004400 0x400>;
+	interrupts = <38>;
+	clocks = <&clk_pclk1>;
+	st,hw-flow-ctrl;
+	pinctrl-names = "default";
+	pinctrl-0 = <&pinctrl_usart2 &pinctrl_usart2_rtscts>;
+};