api/spi: Add a dedicated Kconfig option for asynchronous mode enablement

Instead of using CONFIG_POLL, which is not directly related to SPI and
is a kernel option, let's have SPI_ASYNC instead. When enabled, it will
select POLL automatically.

Fixes #5839

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