rockchip: dts: rk3399: change sd-card io voltage to 3.0V

The VCC_SD and VCC_SDIO rail should only be powered up to 3.0V on RK3399
platforms.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>