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

Move the devicetree binding documentation to the interrupt-controller
directory, where it belongs.

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Link: https://lkml.kernel.org/r/1400457737-1617-1-git-send-email-ezequiel.garcia@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>