spi: Add a utility pointer for the application on SPI callback

It might be needed sometime to get some private data forwarded to the
callback, thus such user_data pointer is the easiest way to proceed.
Adding a macro to set the word size as well.

Change-Id: I68cbe2d480120253ccb13f13c656a38c27e21604
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>