boards: nrf9160dk: Enable external flash switch in board controller

Enable the external_flash_pins_routing switch in the board controller
overlay for nRF9160 DK revision 0.14.0 and newer. This is to match the
behaviour of the factory shipped borad controller on board revisions
that have external flash.

Signed-off-by: Gregers Gram Rygg <gregers.gram.rygg@nordicsemi.no>