spi/imx: remove autodetection

There are no machines in-tree that still use the driver
name as device name.  So save a few bytes and remove it.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>