api/spi: Change transceive functions signature

Instead of NULL terminated buffer arrays, let's add a parameter for each
that tells the number of spi_buf in it.

It adds a little bit more complexity in driver's side (spi_context.h)
but not on user side (bufer one has to take care of providing the NULL
pointer at the end of the array, now he requires to give the count).

This will saves a significant amount of bytes in more complex setup than
the current dumb spi driver sample.

Fix and Use size_t everywhere (spi_context.h was using u32_t).

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