serial: 8250_uniphier: add bindings document for UniPhier UART

This is binding information for the UniPhier on-chip UART driver
(drivers/tty/serial/8250/8250_uniphier.c).
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

+UniPhier UART controller
+
+Required properties:
+- compatible: should be "socionext,uniphier-uart".
+- reg: offset and length of the register set for the device.
+- interrupts: a single interrupt specifier.
+- clocks: phandle to the input clock.
+
+Optional properties:
+- fifo-size: the RX/TX FIFO size.  Defaults to 64 if not specified.
+
+Example:
+	aliases {
+		serial0 = &serial0;
+	};
+
+	serial0: serial@54006800 {
+		compatible = "socionext,uniphier-uart";
+		reg = <0x54006800 0x40>;
+		interrupts = <0 33 4>;
+		clocks = <&uart_clk>;
+		fifo-size = <64>;
+	};