ARM: dts: sun9i: Add NMI controller device node

The Allwinner A80 SoC has an NMI controller. NMI is an external
interrupt pin exclusely used with PMICs and other system critical
peripherals (such as RTC) in Allwinner's reference designs.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>