drivers/spi: Handle synchronous calls in a generic manner in DW driver

All SPI drivers have this same way to handle synchronous call, thus
let's generalize it in struct spi_context, with a relevant API and apply
the change into SPI DW driver.

spi_context API will prove to be useful once asynchronous call will be
handled as well, through the same completion functions used now only for
synchronous call. It will be transparent for the driver.

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