dt-bindings: document the MPS2 UART bindings

This adds documentation of device tree bindings for the
UART found on ARM MPS2 platform
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/arm,mps2-uart.txt

+ARM MPS2 UART
+
+Required properties:
+- compatible	: Should be "arm,mps2-uart"
+- reg		: Address and length of the register set
+- interrupts	: Reference to the UART RX, TX and overrun interrupts
+
+Required clocking property:
+- clocks	  : The input clock of the UART
+
+
+Examples:
+
+uart0: serial@40004000 {
+	compatible = "arm,mps2-uart";
+	reg = <0x40004000 0x1000>;
+	interrupts = <0 1 12>;
+	clocks = <&sysclk>;
+};