drivers: spi_nrfx_spim: Change the way the nrfx driver is configured

According to the nRF5340 PS, for 32 Mbps high-speed SPI using SPIM4,
drive configuration H0H1 must be used. The underlying nrfx_spim driver
does it properly in its initialization function, so change the shim to
(re)initialize the driver when the SPI configuration is to be changed
(only then the speed to use is known), to avoid the need of duplicating
the corresponding code in the shim itself.

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