drivers: spi: nrfx_spim: prevent self suspend until spi_release()

In case the SPI transaction has SPI_HOLD_ON_CS set, we need to keep
SPI resumed until spi_release() is called. This is required as we
now need to keep the CS GPIO port resumed until transaction is
complete.

Suspending CS GPIO is not allowed from ISR in some cases (H20 fast
GPIO instance) so we have to defer CS GPIO suspend to some thread
context (put_async or spi_release()).

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>