bcm2835_mbox_set_power_state
int bcm2835_mbox_set_power_state(uint32_t, boolean_t);
err = bcm2835_mbox_set_power_state(BCM2835_MBOX_POWER_ID_EMMC, TRUE);
err = bcm2835_mbox_set_power_state(BCM2835_MBOX_POWER_ID_USB_HCD, TRUE);