dt-bindings/interrupt-controller: Add documentation for Marvell SEI controller

Describe the System Error Interrupt (SEI) controller. It aggregates two
types of interrupts, wired and MSIs from respectively the AP and the
CPs, into a single SPI interrupt.
Suggested-by: Haim Boot <hayim@marvell.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Documentation/devicetree/bindings/interrupt-controller/marvell,sei.txt

+Marvell SEI (System Error Interrupt) Controller
+-----------------------------------------------
+
+Marvell SEI (System Error Interrupt) controller is an interrupt
+aggregator. It receives interrupts from several sources and aggregates
+them to a single interrupt line (an SPI) on the parent interrupt
+controller.
+
+This interrupt controller can handle up to 64 SEIs, a set comes from the
+AP and is wired while a second set comes from the CPs by the mean of
+MSIs.
+
+Required properties:
+
+- compatible: should be one of:
+              * "marvell,ap806-sei"
+- reg: SEI registers location and length.
+- interrupts: identifies the parent IRQ that will be triggered.
+- #interrupt-cells: number of cells to define an SEI wired interrupt
+                    coming from the AP, should be 1. The cell is the IRQ
+                    number.
+- interrupt-controller: identifies the node as an interrupt controller
+                        for AP interrupts.
+- msi-controller: identifies the node as an MSI controller for the CPs
+                  interrupts.
+
+Example:
+
+        sei: interrupt-controller@3f0200 {
+                compatible = "marvell,ap806-sei";
+                reg = <0x3f0200 0x40>;
+                interrupts = <GIC_SPI 0 IRQ_TYPE_LEVEL_HIGH>;
+                #interrupt-cells = <1>;
+                interrupt-controller;
+                msi-controller;
+        };