dt-bindings: fsi: Add description of FSI master

This describes the FSI master present in the AST2600.
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Alistair Popple <alistair@popple.id.au>
Link: https://lore.kernel.org/r/20191108051945.7109-9-joel@jms.id.auSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/fsi/fsi-master-aspeed.txt

+Device-tree bindings for AST2600 FSI master
+-------------------------------------------
+
+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>;
+    };