drivers: serial: nrfx: Clean up driver instantiation

- use CONFIG_HAS_HW_NRF_* symbols consistently in nRF multi-instance
  drivers when creating particular driver instances
- remove unnecessary hidden Kconfig options that indicated the type of
  peripheral to be used by a given instance (e.g. SPI, SPIM, or SPIS)
  and enabled proper nrfx driver instance; instead, use one option per
  peripheral type and include the corresponding shim driver flavor into
  compilation basing on that option (not the one that enables the nrfx
  driver as it was incorrectly done so far in some cases)

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