[nrf fromtree] drivers: mspi_dw: Add support for asynchronous transfers

Handling of asynchronous transfers uses the system workqueue,
hence they are not available when multithreading is disabled.

Also add missing dependency on multithreading in the
MSPI_DW_HANDLE_FIFOS_IN_SYSTEM_WORKQUEUE Kconfig option.

Signed-off-by: David Jewsbury <[email protected]>
Signed-off-by: Andrzej Głąbek <[email protected]>
(cherry picked from commit 2f1ee73)

Author: Hi-Im-David

drivers/mspi/Kconfig.dw

@@ -14,6 +14,7 @@ if MSPI_DW
 
 config MSPI_DW_HANDLE_FIFOS_IN_SYSTEM_WORKQUEUE
 	bool "Handle FIFO in system workqueue"
+	depends on MULTITHREADING
 	help
 	  When the driver does not use DMA for transferring data to/from the
 	  SSI FIFOs, handling of those may take a significant amount of time.