irqchip: armada-370-xp: Move the devicetree binding documentation

Move the devicetree binding documentation to the interrupt-controller
directory, where it belongs.
Reviewed-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Link: https://lkml.kernel.org/r/1400457737-1617-1-git-send-email-ezequiel.garcia@free-electrons.comSigned-off-by: NJason Cooper <jason@lakedaemon.net>