dt-bindings: serial: Document RDA Micro UART

Add an initial binding for the UART in RDA Micro RDA8810PL SoC.
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>

Documentation/devicetree/bindings/serial/rda,8810pl-uart.txt

+RDA Micro UART
+
+Required properties:
+- compatible :  "rda,8810pl-uart" for RDA8810PL SoCs.
+- reg        :  Offset and length of the register set for the device.
+- interrupts :  Should contain UART interrupt.
+- clocks     :  Phandle to the input clock.
+
+
+Example:
+
+		uart2: serial@20a90000 {
+			compatible = "rda,8810pl-uart";
+			reg = <0x20a90000 0x1000>;
+			interrupts = <11 IRQ_TYPE_LEVEL_HIGH>;
+			clocks = <&uart_clk>;
+		};