Remove the empty spi_imx_cleanup function.

It's ok if a driver does not set the controller->cleanup pointer, the
caller does a NULL check.
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://msgid.link/r/20240520165906.164906-1-martin@kaiser.cxSigned-off-by: Mark Brown <broonie@kernel.org>