drivers: flash: nrf_qspi_nor: support specifying only 2 io-pins

Currently user is forced to configure an array of 4 IO pins. This makes
no sense when there are only 2 IO pins connected on board.

Configure 3rd and 4th pin in internal structure as
NRF_QSPI_PIN_NOT_CONNECTED if only 2 were specified in device-tree.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>