ARM: imx: change the way spi-imx devices are registered

Group soc specific data in a global struct instead of repeating it for
each call to imxXX_add_spi_imxX.  The structs holding the actual data
are placed in .init.constdata and so don't do much harm.  Compared to
the previous approach this reduces code size to call imx_add_spi_imx.

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>