arm64: dts: allwinner: h6: Add LED device nodes for Pine H64

The Pine H64 has 3 GPIO-controlled LEDs, which are labeled "heartbeat",
"link", and "status".

Add device nodes for them.

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