ARM: sun7i/sun6i: irqchip: Update the documentation

Add documentation for NMI irqchip.

Signed-off-by: Carlo Caione <carlo@caione.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-sunxi@googlegroups.com
Cc: mark.rutland@arm.com
Cc: hdegoede@redhat.com
Acked-by:  <maxime.ripard@free-electrons.com>
Link: http://lkml.kernel.org/r/1395256879-8475-4-git-send-email-carlo@caione.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>