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: Damien Riegel <damien.riegel@savoirfairelinux.com>
Acked-by: Rob 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.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>