drivers: spi: nrfx: Remove "optional" from driver type choices

Selecting the type of driver (SPI, SPIM, or SPIS) to be used for
a given SPI instance is mandatory, so the "optional" modifier is
inappropriate for related choice options in the driver's Kconfig.

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