ARM: dts: sun8i: add a cpu0 label to cpu@0 node on A23/33

A "cpu0" label is needed on cpu@0 for cpufreq-dt to work.

Add such a label, in order to prepare for cpufreq support of A23/33.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>