As I understand KConfig if CONFIG_MFD_AXP20X were not set there would be no CONFIG_INPUT_AXP20X_PEK line in the config. Also the poweroff command would not power off the board because it's the AXP20X that drives the CPU regulators and turns off the power on shutdown. The DT part should be there because in general the DT bindings for the hardware are merged together with the driver. Should it not be the case it's another board-specific problem.