samples: boards: nrfx: Use absolute pin numbers

This sample makes calls to the nrfx_gpiote driver functions that
require absolute pin numbers, not relative to a given GPIO port,
so instead of getting the numbers with the `DT_GPIO_PIN` macro,
it should use `NRF_DT_GPIOS_TO_PSEL`.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>