Write this in terms of DT_GPIO_CTLR_BY_IDX instead of the lower level DT_PHANDLE_BY_IDX for clarity and parallelism with things like DT_SPI_DEV_CS_GPIOS_PIN, which is written using DT_GPIO_PIN_BY_IDX instead of DT_PHA_BY_IDX. Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>