spi: Use after free fixes

Johan Hovold <johan@kernel.org> says:

The SPI subsystem frees the controller and any subsystem allocated
driver data as part of deregistration (unless the allocation is device
managed).

This series fixes the IMX driver that got this wrong and then converts
it to use device managed allocation.

Included are also a (preparatory) deregistration fix for the rockchip
driver and related cleanups for the tegre20-slink and rockchip drivers
that both take a controller reference during unbind.