samples: button: Fix button sample GPIO for nRF52

Applies to #4008 and #5159 for this board. Problem is that the
button interrupt callback was only firing once. Solution is to
set the pin pull up flag to GPIO_PUD_PULL_UP.

Signed-off-by: Anders Pitman <tapitman11@gmail.com>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>