ARM: dts: sunxi: Fix GIC compatible

As can be shown by the YAML schema now, the combination of GIC compatibles
we were using has never been an option.

Switch to the gic-400 variant, which is the more correct option.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>