spi: imx: put struct spi_imx_config members into driver private struct

struct spi_imx_config used to hold data specific to the current
transfer. However, other data is in the drivers private data struct.
Let's drop struct spi_imx_config and put the variables into the
drivers private data struct aswell.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>