soc: nrf: add more DT GPIO helper macros

Add some helper macros that will be convenient to use from device
drivers for accessing and error checking pin mux information in the
devicetree:

- NRF_DT_PSEL(): get a PSEL value out of the DT from either a
  'foo-pin' or a 'foo-gpios' style property.

- NRF_DT_PSEL_CHECK_NOT_BOTH(), NRF_DT_PSEL_CHECK_EXACTLY_ONE():
  helpers for checking that a given devicetree is OK according to
  different criteria for setting PSEL properties (NAND or XOR on
  whether the properties exist, respectively).

See comments in the patch for more details.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>