Phytium INTx interrupt controller (IXIC)

This is a pseudo interrupt controller to handle PCI legacy interrupt on
Phytium D2000 and FT-2000/4C SoC, which sits between the PCI INTx devices
and the GIC and forwards the 4 INTx input signals to 4 adjacent GICv3 SPIs.

Required properties:

- compatible		: "phytium,d2000-ixic"
			  "phytium,ft2004c-ixic"
- reg			: Specifies two regions of the register set, which
			  are called 'ctr' and 'hpb'.
- interrupt-controller	: Identifies the node as an interrupt controller.
- #interrupt-cells	: Specifies the number of cells needed to encode an
			  interrupt source. The value must be 3.
- intx-spi-base		: The SPI number of the first SPI of the 4 adjacent
			  ones the IXIC forwards its interrupts to.

Example:
	ixic: interrupt-controller@29000000 {
		compatible = "phytium,d2000-ixic";
		reg-names = "ctr", "hpb";
		reg = <0x0 0x29000000 0x0 0x00060000>,
		      <0x0 0x29100000 0x0 0x00002000>;
		interrupt-controller;
		interrupt-parent = <&gic>;
		#interrupt-cells = <3>;
		intx-spi-base = <28>;
	};
