In this example SW0_GPIO_FLAGS is not "consumed" by any source code, but "samples/basic/button/src/main.c" application is using SW0_GPIO_CONTROLLER, SW0_GPIO_PIN and SW0_GPIO_INT_CONF.
I think we should name this gpio dts cell as "int_conf" instead of "flags", this way the dts configuration will be used in the button application, for example.