Documentation: DT: Add entries for bcm63xx UART

This squashes a checkpatch warning on my new bcm3384 dts submission.
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/bcm63xx-uart.txt

+* 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>;
+		};
+	};