arm64: dts: allwinner: h5: Add cpu0 label for first cpu

At the board level, we want to be able to specify what regulator
supplies power to the cpu domain.

Add a label to the first cpu node so we can reference it later.

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