samples/boards/nrf/system_off: Fix configuration of the wake up button

Use NRF_DT_GPIOS_TO_PSEL() in calls to nRF GPIO HAL functions as they
need a psel value, a pin number combined with the corresponding GPIO
port number, not only the pin number as provided by DT_GPIO_PIN().
This way buttons connected to pins in the P1 port can also be handled
properly.
Replace also DT_NODELABEL(button0) with DT_ALIAS(sw0) for consistency
with other samples that use the standard button.

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