The sdhci should be able to control the regulator (gpio) from software.
Do you put your gpio pin to the device tree?
→ /t18x-common-platforms/tegra186-quill-power-tree-p3310-1000-a00-00.dtsi
sdhci@3400000 {
vqmmc-supply = <&spmic_ldo3>;
vmmc-supply = <&en_vdd_sdcard1>;
};