rockchip: evb-rk3399: Provide init voltage

Add missing regulator-init-microvolt property to vdd_center regulator.
Signed-off-by: NKever Yang <kever.yang@rock-chips.com>

arch/arm/dts/rk3399-evb-u-boot.dtsi

@@ -38,6 +38,10 @@
 	status = "okay";
 };
 
+&vdd_center {
+	regulator-init-microvolt = <900000>;
+};
+
 &sdmmc {
 	u-boot,dm-pre-reloc;
 	bus-width = <4>;