drivers/gpio/gpio_rzt2m: Handle trig == GPIO_INT_TRIG_WAKE

Fix a build warning due to the enumerate for the trigger
polarity possibly being (from the point of the compiler)
also GPIO_INT_TRIG_WAKE.
We fix it by simply returning a not valid argument error
which is what most other drivers do if they do something
smart enough.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>