drivers: spi: Add shim for nrfx SPI driver

This adds a translation layer to make the nrfx driver for the legacy
(i.e. without EasyDMA) nRF SPI peripheral accessible via the updated
Zephyr's API of the SPI driver.
Configuration files are already prepared for adding support for SPIM
(Master with EasyDMA) and SPIS (Slave with EasyDMA) peripherals.

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