spi/imx: copy gpio number passed by platform data into driver private data

It copies gpio number passed via platform data embedded pointer into
driver private data, so that we do not need to refer to this embedded
pointer passed by platform data after probe function exits.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>