irqchip/ts4800: Add documentation for TS-4800 interrupt controller

This is an interrupt-controller implemented in an FPGA, to multiplex
interrupts generated from other IPs. The FPGA usually uses a GPIO as a
parent interrupt controller to notify that one of the multiplexed
interrupts has triggered.
Signed-off-by: NDamien Riegel <damien.riegel@savoirfairelinux.com>
Acked-by: NRob Herring <robh@kernel.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: kernel@savoirfairelinux.com
Link: http://lkml.kernel.org/r/1450728683-31416-1-git-send-email-damien.riegel@savoirfairelinux.comSigned-off-by: NThomas Gleixner <tglx@linutronix.de>

Documentation/devicetree/bindings/interrupt-controller/technologic,ts4800.txt

+TS-4800 FPGA interrupt controller
+
+TS-4800 FPGA has an internal interrupt controller. When one of the
+interrupts is triggered, the SoC is notified, usually using a GPIO as
+parent interrupt source.
+
+Required properties:
+- compatible: should be "technologic,ts4800-irqc"
+- interrupt-controller: identifies the node as an interrupt controller
+- reg: physical base address of the controller and length of memory mapped
+  region
+- #interrupt-cells: specifies the number of cells needed to encode an interrupt
+  source, should be 1.
+- interrupt-parent: phandle to the parent interrupt controller this one is
+  cascaded from
+- interrupts: specifies the interrupt line in the interrupt-parent controller