rockchip: rk3399: rock-pi4: Add init value for vdd_log

We should set the init value when vdd_log is enabled, or else the
vdd_log output voltage may not in soc required range.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>

arch/arm/dts/rk3399-rock-pi-4-u-boot.dtsi

@@ -11,3 +11,7 @@
 		u-boot,spl-boot-order = "same-as-spl", &sdhci, &sdmmc;
 	};
 };
+
+&vdd_log {
+	regulator-init-microvolt = <950000>;
+};