dt/bindings: serial: bcm2835: add binding documentation for bcm2835-aux-uart

Add binding documentation for the bcm2835-aux-uart driver.
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/brcm,bcm2835-aux-uart.txt

+* BCM2835 AUXILIAR UART
+
+Required properties:
+
+- compatible: "brcm,bcm2835-aux-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:
+
+	uart1: serial@7e215040 {
+		compatible = "brcm,bcm2835-aux-uart";
+		reg = <0x7e215040 0x40>;
+		interrupts = <1 29>;
+		clocks = <&aux BCM2835_AUX_CLOCK_UART>;
+	};