arm64: dts: allwinner: h6: Use - instead of @ for DT OPP entries

DTC and the dt-validate tools report warnings for opp with the format
opp@$frequency: dtc for a missing reg property, and dt-validate since
the binding requires child nodes to have the format opp-$frequency.

Change this to the latter format.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://lore.kernel.org/r/20210114113538.1233933-16-maxime@cerno.tech