Seems like there is a bug or two in the kernel.
Mediatek now know about it, but no one has responded yet as to a fix.
FWIW, you need CONFIG_PCIE_MEDIATEK_GEN3, CONFIG_REGULATOR_MT6380, CONFIG_MTK_PMIC_WRAP and CONFIG_PHY_MTK_TPHY all baked into the kernel rather than as modules.