drivers: spi: spi_nrfx_spim: Remove nrf_frequency_t handling

With new nrfx release SPIM driver stores frequency as a `uint32_t`
type representing frequency in `Hz` in its configuration structure.
Additionally `NRFX_SPIM_PIN_NOT_USED` has been removed,
`NRF_SPIM_PIN_NOT_CONNECTED` symbol is used instead.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>