arm64: dts: allwinner: a64: NanoPi-A64: Add blue status LED

Beside the non-controllable green power LED, the NanoPi-A64 features a
blue "status" LED, connected to PD24.

Add the device tree node to make it usable.

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