api: Add asynchronous call support to SPI API

Adding a struct k_poll_signal parameter to driver's API unique
exposed function.

If not NULL, the call will be handled as asynchronous and will
return right after the transaction has started, on the contrary
of current logic where is waits for the transaction to finish
(= synchronous).

In order to save stack, let's move the device pointer to struct
spi_config. So the call is still at a maximum of 4 parameters.

Adapting spi_dw.c and spi driver sample to the change so it still
builts.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>