dt/bindings: Add bindings for Layerscape external irqs

This adds Device Tree binding documentation for the external interrupt
lines with configurable polarity present on some Layerscape SOCs.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20191107122115.6244-2-linux@rasmusvillemoes.dk

Documentation/devicetree/bindings/interrupt-controller/fsl,ls-extirq.txt

+* Freescale Layerscape external IRQs
+
+Some Layerscape SOCs (LS1021A, LS1043A, LS1046A) support inverting
+the polarity of certain external interrupt lines.
+
+The device node must be a child of the node representing the
+Supplemental Configuration Unit (SCFG).
+
+Required properties:
+- compatible: should be "fsl,<soc-name>-extirq", e.g. "fsl,ls1021a-extirq".
+- #interrupt-cells: Must be 2. The first element is the index of the
+  external interrupt line. The second element is the trigger type.
+- #address-cells: Must be 0.
+- interrupt-controller: Identifies the node as an interrupt controller
+- reg: Specifies the Interrupt Polarity Control Register (INTPCR) in
+  the SCFG.
+- interrupt-map: Specifies the mapping from external interrupts to GIC
+  interrupts.
+- interrupt-map-mask: Must be <0xffffffff 0>.
+
+Example:
+	scfg: scfg@1570000 {
+		compatible = "fsl,ls1021a-scfg", "syscon";
+		reg = <0x0 0x1570000 0x0 0x10000>;
+		big-endian;
+		#address-cells = <1>;
+		#size-cells = <1>;
+		ranges = <0x0 0x0 0x1570000 0x10000>;
+
+		extirq: interrupt-controller@1ac {
+			compatible = "fsl,ls1021a-extirq";
+			#interrupt-cells = <2>;
+			#address-cells = <0>;
+			interrupt-controller;
+			reg = <0x1ac 4>;
+			interrupt-map =
+				<0 0 &gic GIC_SPI 163 IRQ_TYPE_LEVEL_HIGH>,
+				<1 0 &gic GIC_SPI 164 IRQ_TYPE_LEVEL_HIGH>,
+				<2 0 &gic GIC_SPI 165 IRQ_TYPE_LEVEL_HIGH>,
+				<3 0 &gic GIC_SPI 167 IRQ_TYPE_LEVEL_HIGH>,
+				<4 0 &gic GIC_SPI 168 IRQ_TYPE_LEVEL_HIGH>,
+				<5 0 &gic GIC_SPI 169 IRQ_TYPE_LEVEL_HIGH>;
+			interrupt-map-mask = <0xffffffff 0x0>;
+		};
+	};
+
+
+	interrupts-extended = <&gic GIC_SPI 88 IRQ_TYPE_LEVEL_HIGH>,
+			      <&extirq 1 IRQ_TYPE_LEVEL_LOW>;