arm64: dts: allwinner: h6: add voltage range to OPP table

Some boards have a fixed regulator and can't reach the voltage set
by the OPP table.

Add a range where the minimal voltage is the target and the maximal
voltage is 1.2V.

Suggested-by: Ondřej Jirman <megous@megous.com>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>