ARM: imx: use platform ids for spi_imx devices

The driver recently learned to handle platform ids.  Make use of this
new feature.  The up side is that the driver needs less knowledge about
the spi interfaces used on different SoCs.

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