spi: spi-imx: replace struct spi_imx_data::bitbang by pointer to struct spi_controller

spi: spi-imx: replace struct spi_imx_data::bitbang by pointer to struct spi_controller

There's no need to embed the struct spi_bitbang into our private
data (struct spi_imx_data), the spi core is flexible enough, so that
we only need a pointer to the allocated struct spi_controller.

This is also a preparation patch to add PIO based polling support to
the driver.

Co-developed-by: David Jander <david@protonic.nl>
Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20220502175457.1977983-8-mkl@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>