spi/spi-fsl-spi: Introduce a type for the driver

For being able to distinguishing between the regular type of cores and others
with different entries in of_fsl_spi_match.

Acked-by: Anton Vorontsov <anton@enomsg.org>
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>