* 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.
- dmas: phandle(s) to DMA controller node(s). Refer to stm32-dma.txt
- dma-names: "rx" and/or "tx"

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>;
};

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";
};