drivers: spi: spi_nrfx_spim: Include nrf_clock.h only for nRF5340

The CLOCK HAL header is only needed for nRF5340 SoC. It's used when
user wants to configure SPIM instance to 32 Mbps. The HAL checks
if is running at 128 MHz as only then 32 Mbps is supported.

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