boards: nrf9160dk: Move board control configuration to devicetree

Use devicetree instead of Kconfig to configure the board control
switches in nRF9160 DK:
- add binding for the switches that provide optional signal routings
  on this board
- add binding for the GPIO interface that can be used for communication
  (e.g. UART based) between the nRF9160 and the nRF52840 on the DK,
  and add GPIO mapping for this interface so that its lines can be used
  without caring about of actual pin numbers on both sides
- add binding for one GPIO line chosen from the above interface that is
  to allow the nRF9160 to reset the nRF52840
- update accordingly dts files and board specific code for both board
  definitions associated with the DK
- introduce .dtsi files that can be included from dts overlays in order
  to facilitate the use of the above GPIO interface; modify the overlay
  in the hci_uart sample to provide an example of use of those files

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