ARM: dts: sunxi: Fix OPPs node name

The operating-points-v2 nodes are named inconsistently, but mostly
either opp_table0 or gpu-opp-table.  However, the underscore is an
invalid character for a node name and the thermal zone binding
explicitly requires that zones are called opp-table-*. Let's fix it.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20210901091852.479202-43-maxime@cerno.tech