dts: Use separate compatibles for Nordic SPI/SPIM/SPIS peripherals

This commit introduces separate "compatible" strings for DTS nodes
representing different types of Nordic SPI peripherals. Previously
"nordic,nrf-spi" was used for both SPI and SPIM. SPIS was already
handled separately.

Quite a few files need to be touched by this commit but the changes can
be divided into groups of related or very similar ones, distinguishable
by the initial part of the path to the modified file:

* dts/bindings/spi/
  new binding for "nordic,nrf-spim" is added and common fields for all
  3 types of Nordic SPI peripherals are extracted to a shared file

* dts/arm/nordic/
  "compatible" properties in spiX nodes are updated (when there is no
  choice as only one type of SPI peripheral is available) or replaced
  with a comment pointing out that the proper type of peripheral needs
  to be picked at some upper layer

* drivers/spi/
  spi_nrfx_spim driver is updated with the new form of macros generated
  from dts

* boards/
  all spiX nodes in dts files for boards equipped with an nRF chip are
  updated with the proper "compatible" property, according to the type
  of SPI peripheral that is currently selected for the board by the
  corresponding Kconfig choice option (SPI_x_NRF_SPI*)

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