spi/imx: save the spi chip select in config struct, not the gpio to use

This prepares adding support for imx51's eCSPI.  This IP has seperate
control and config bits for all four supported chip selects, so the
config routine needs to know which chip select is being used even if
the chipselect is realized by a gpio.

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>