serial: 8250_uniphier: add bindings document for UniPhier UART

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>;

	};