arm64: dts: rockchip: Update CPU regulator voltage ranges for Gru

Gru derivatives besides Kevin have slightly different voltage ranges for
their CPU regulators. Let's keep the base Gru file accurate and let
Kevin override.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>