arm64: dts: rockchip: correct voltage range for rk3368-evb-act8846 board

In general, the logic voltage is affected by ddr frequency factors.
We should fix the correct voltage range since assuemd that we have the
ddr frequency driver in mainline.

AFAIK, the 1.8v voltage is used by the SD3.0 card.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>