dt-bindings: fsi: Add description of FSI master

Device-tree bindings for AST2600 FSI master

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The AST2600 contains two identical FSI masters. They share a clock and have a

separate interrupt line and output pins.

Required properties:

 - compatible: "aspeed,ast2600-fsi-master"

 - reg: base address and length

 - clocks: phandle and clock number

 - interrupts: platform dependent interrupt description

 - pinctrl-0: phandle to pinctrl node

 - pinctrl-names: pinctrl state

Examples:

    fsi-master {

        compatible = "aspeed,ast2600-fsi-master", "fsi-master";

        reg = <0x1e79b000 0x94>;

	interrupts = <GIC_SPI 100 IRQ_TYPE_LEVEL_HIGH>;

	pinctrl-names = "default";

	pinctrl-0 = <&pinctrl_fsi1_default>;

	clocks = <&syscon ASPEED_CLK_GATE_FSICLK>;

    };