arm64: dts: rockchip: used range'd gpu opps on rk3399

Similar to the cpu opps, also use opps with a range on the gpu.
(min, preferred, max). The voltage just needs to be higher than
the minimum and this allows the regulator more freedom if it
can't provide the exact voltage specified, but just say 5mV higher,
as can be seen on rk3399-puma which fails to scale panfrost voltages
nearly completely.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Link: https://lore.kernel.org/r/20210225133322.3420724-2-heiko@sntech.de


Signed-off-by: Heiko Stuebner <heiko@sntech.de>