SiFive asynchronous serial interface (UART)

Required properties:

- compatible: should be something similar to
	      "sifive,<chip>-uart" for the UART as integrated
	      on a particular chip, and "sifive,uart<version>" for the
	      general UART IP block programming model.	Supported
	      compatible strings as of the date of this writing are:
	      "sifive,fu540-c000-uart" for the SiFive UART v0 as
	      integrated onto the SiFive FU540 chip, or "sifive,uart0"
	      for the SiFive UART v0 IP block with no chip integration
	      tweaks (if any)
- reg: address and length of the register space
- interrupts: Should contain the UART interrupt identifier
- clocks: Should contain a clock identifier for the UART's parent clock


UART HDL that corresponds to the IP block version numbers can be found
here:

https://github.com/sifive/sifive-blocks/tree/master/src/main/scala/devices/uart


Example:

uart0: serial@10010000 {
	compatible = "sifive,fu540-c000-uart", "sifive,uart0";
	interrupt-parent = <&plic0>;
	interrupts = <80>;
	reg = <0x0 0x10010000 0x0 0x1000>;
	clocks = <&prci PRCI_CLK_TLCLK>;
};