Documentation: DT: Add entries for bcm63xx UART

* BCM63xx UART

Required properties:

- compatible: "brcm,bcm6345-uart"

- reg: The base address of the UART register bank.

- interrupts: A single interrupt specifier.

- clocks: Clock driving the hardware; used to figure out the baud rate

  divisor.

Example:

	uart0: serial@14e00520 {

		compatible = "brcm,bcm6345-uart";

		reg = <0x14e00520 0x18>;

		interrupt-parent = <&periph_intc>;

		interrupts = <2>;

		clocks = <&periph_clk>;

	};

	clocks {

		periph_clk: periph_clk@0 {

			compatible = "fixed-clock";

			#clock-cells = <0>;

			clock-frequency = <54000000>;

		};

	};